diff --git a/Source/StrongGrid/ILegacyClient.cs b/Source/StrongGrid/ILegacyClient.cs
index 75685577..8ecf6a4f 100644
--- a/Source/StrongGrid/ILegacyClient.cs
+++ b/Source/StrongGrid/ILegacyClient.cs
@@ -1,8 +1,11 @@
+using System;
+
 namespace StrongGrid
 {
 	/// <summary>
 	/// Interface for the SendGrid REST client for SendGrid's legacy API.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ILegacyClient
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/LegacyClient.cs b/Source/StrongGrid/LegacyClient.cs
index f30b4dfb..cf631fbe 100644
--- a/Source/StrongGrid/LegacyClient.cs
+++ b/Source/StrongGrid/LegacyClient.cs
@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Logging;
 using StrongGrid.Utilities;
+using System;
 using System.Net;
 using System.Net.Http;
 
@@ -8,6 +9,7 @@ namespace StrongGrid
 	/// <summary>
 	/// REST client for interacting with SendGrid's legacy API.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class LegacyClient : BaseClient, ILegacyClient
 	{
 		private static readonly StrongGridClientOptions _defaultOptions = new StrongGridClientOptions()
diff --git a/Source/StrongGrid/Models/Legacy/Campaign.cs b/Source/StrongGrid/Models/Legacy/Campaign.cs
index e7d6c46b..1232c900 100644
--- a/Source/StrongGrid/Models/Legacy/Campaign.cs
+++ b/Source/StrongGrid/Models/Legacy/Campaign.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -7,6 +8,7 @@ namespace StrongGrid.Models.Legacy
 	/// you will be required to provide a subject, sender ID, content (we suggest both html
 	/// and plain text), and at least one list or segment ID.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Campaign
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/CampaignStatus.cs b/Source/StrongGrid/Models/Legacy/CampaignStatus.cs
index 1f4c6403..c0a7e59f 100644
--- a/Source/StrongGrid/Models/Legacy/CampaignStatus.cs
+++ b/Source/StrongGrid/Models/Legacy/CampaignStatus.cs
@@ -1,4 +1,5 @@
 using StrongGrid.Json;
+using System;
 using System.Runtime.Serialization;
 using System.Text.Json.Serialization;
 
@@ -7,6 +8,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// Enumeration to indicate the status of a campaign.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	[JsonConverter(typeof(StringEnumConverter<CampaignStatus>))]
 	public enum CampaignStatus
 	{
diff --git a/Source/StrongGrid/Models/Legacy/Contact.cs b/Source/StrongGrid/Models/Legacy/Contact.cs
index b9926df7..b448aee7 100644
--- a/Source/StrongGrid/Models/Legacy/Contact.cs
+++ b/Source/StrongGrid/Models/Legacy/Contact.cs
@@ -9,6 +9,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// A contact (also known as a recipient).
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Contact
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/CustomFieldMetadata.cs b/Source/StrongGrid/Models/Legacy/CustomFieldMetadata.cs
index 4b7523d4..50e0ad17 100644
--- a/Source/StrongGrid/Models/Legacy/CustomFieldMetadata.cs
+++ b/Source/StrongGrid/Models/Legacy/CustomFieldMetadata.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -6,6 +7,7 @@ namespace StrongGrid.Models.Legacy
 	/// Metadata about a custom field.
 	/// </summary>
 	/// <seealso cref="StrongGrid.Models.Legacy.FieldMetadata" />
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class CustomFieldMetadata : FieldMetadata
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/Field.cs b/Source/StrongGrid/Models/Legacy/Field.cs
index f815ac28..c606c590 100644
--- a/Source/StrongGrid/Models/Legacy/Field.cs
+++ b/Source/StrongGrid/Models/Legacy/Field.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -5,6 +6,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// A field.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Field
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/FieldMetadata.cs b/Source/StrongGrid/Models/Legacy/FieldMetadata.cs
index 7d11a252..23da9e04 100644
--- a/Source/StrongGrid/Models/Legacy/FieldMetadata.cs
+++ b/Source/StrongGrid/Models/Legacy/FieldMetadata.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -5,6 +6,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// Metadata about a field.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class FieldMetadata
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/ImportResult.cs b/Source/StrongGrid/Models/Legacy/ImportResult.cs
index 79c9ce52..161bb749 100644
--- a/Source/StrongGrid/Models/Legacy/ImportResult.cs
+++ b/Source/StrongGrid/Models/Legacy/ImportResult.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -5,6 +6,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// The result of importing a group of contacts.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class ImportResult
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/List.cs b/Source/StrongGrid/Models/Legacy/List.cs
index 13830d98..5a089e21 100644
--- a/Source/StrongGrid/Models/Legacy/List.cs
+++ b/Source/StrongGrid/Models/Legacy/List.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -5,6 +6,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// A list of contacts.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class List
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/LogicalOperator.cs b/Source/StrongGrid/Models/Legacy/LogicalOperator.cs
index 5eac9053..2f67e3b5 100644
--- a/Source/StrongGrid/Models/Legacy/LogicalOperator.cs
+++ b/Source/StrongGrid/Models/Legacy/LogicalOperator.cs
@@ -1,4 +1,5 @@
 using StrongGrid.Json;
+using System;
 using System.Runtime.Serialization;
 using System.Text.Json.Serialization;
 
@@ -7,6 +8,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// Enumeration to indicate a logical operator.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	[JsonConverter(typeof(StringEnumConverter<LogicalOperator>))]
 	public enum LogicalOperator
 	{
diff --git a/Source/StrongGrid/Models/Legacy/SearchCondition.cs b/Source/StrongGrid/Models/Legacy/SearchCondition.cs
index 2193f837..08285090 100644
--- a/Source/StrongGrid/Models/Legacy/SearchCondition.cs
+++ b/Source/StrongGrid/Models/Legacy/SearchCondition.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -5,6 +6,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// Search condition.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class SearchCondition
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Models/Legacy/Segment.cs b/Source/StrongGrid/Models/Legacy/Segment.cs
index ab6d40f6..52762c0c 100644
--- a/Source/StrongGrid/Models/Legacy/Segment.cs
+++ b/Source/StrongGrid/Models/Legacy/Segment.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Text.Json.Serialization;
 
 namespace StrongGrid.Models.Legacy
@@ -5,6 +6,7 @@ namespace StrongGrid.Models.Legacy
 	/// <summary>
 	/// Segment.
 	/// </summary>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Segment
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/Campaigns.cs b/Source/StrongGrid/Resources/Legacy/Campaigns.cs
index 53be7dd0..a548fb97 100644
--- a/Source/StrongGrid/Resources/Legacy/Campaigns.cs
+++ b/Source/StrongGrid/Resources/Legacy/Campaigns.cs
@@ -17,6 +17,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/campaigns.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Campaigns : ICampaigns
 	{
 		private const string _endpoint = "campaigns";
diff --git a/Source/StrongGrid/Resources/Legacy/Categories.cs b/Source/StrongGrid/Resources/Legacy/Categories.cs
index cbab43c3..d9448a6a 100644
--- a/Source/StrongGrid/Resources/Legacy/Categories.cs
+++ b/Source/StrongGrid/Resources/Legacy/Categories.cs
@@ -1,4 +1,5 @@
 using Pathoschild.Http.Client;
+using System;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -12,6 +13,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Categories/categories.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Categories : ICategories
 	{
 		private const string _endpoint = "categories";
diff --git a/Source/StrongGrid/Resources/Legacy/Contacts.cs b/Source/StrongGrid/Resources/Legacy/Contacts.cs
index 948b93e4..bf621224 100644
--- a/Source/StrongGrid/Resources/Legacy/Contacts.cs
+++ b/Source/StrongGrid/Resources/Legacy/Contacts.cs
@@ -19,6 +19,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/contactdb.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Contacts : IContacts
 	{
 		private const string _endpoint = "contactdb/recipients";
diff --git a/Source/StrongGrid/Resources/Legacy/CustomFields.cs b/Source/StrongGrid/Resources/Legacy/CustomFields.cs
index d5a99668..9075429c 100644
--- a/Source/StrongGrid/Resources/Legacy/CustomFields.cs
+++ b/Source/StrongGrid/Resources/Legacy/CustomFields.cs
@@ -1,6 +1,7 @@
 using Pathoschild.Http.Client;
 using StrongGrid.Json;
 using StrongGrid.Models;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -13,6 +14,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/contactdb.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class CustomFields : ICustomFields
 	{
 		private const string _endpoint = "contactdb/custom_fields";
diff --git a/Source/StrongGrid/Resources/Legacy/ICampaigns.cs b/Source/StrongGrid/Resources/Legacy/ICampaigns.cs
index 82d4cfc7..f9259b77 100644
--- a/Source/StrongGrid/Resources/Legacy/ICampaigns.cs
+++ b/Source/StrongGrid/Resources/Legacy/ICampaigns.cs
@@ -13,6 +13,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/campaigns.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ICampaigns
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/ICategories.cs b/Source/StrongGrid/Resources/Legacy/ICategories.cs
index d1074d7a..3d0a4990 100644
--- a/Source/StrongGrid/Resources/Legacy/ICategories.cs
+++ b/Source/StrongGrid/Resources/Legacy/ICategories.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -9,6 +10,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Categories/categories.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ICategories
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/IContacts.cs b/Source/StrongGrid/Resources/Legacy/IContacts.cs
index 266ecb65..dd8e6902 100644
--- a/Source/StrongGrid/Resources/Legacy/IContacts.cs
+++ b/Source/StrongGrid/Resources/Legacy/IContacts.cs
@@ -1,5 +1,6 @@
 using StrongGrid.Models;
 using StrongGrid.Utilities;
+using System;
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -12,6 +13,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/contactdb.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface IContacts
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/ICustomFields.cs b/Source/StrongGrid/Resources/Legacy/ICustomFields.cs
index 559ac563..9fb0957e 100644
--- a/Source/StrongGrid/Resources/Legacy/ICustomFields.cs
+++ b/Source/StrongGrid/Resources/Legacy/ICustomFields.cs
@@ -1,4 +1,5 @@
 using StrongGrid.Models;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -10,6 +11,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/contactdb.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ICustomFields
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/ILists.cs b/Source/StrongGrid/Resources/Legacy/ILists.cs
index 0f54cca1..62d72b42 100644
--- a/Source/StrongGrid/Resources/Legacy/ILists.cs
+++ b/Source/StrongGrid/Resources/Legacy/ILists.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -10,6 +11,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/contactdb.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ILists
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/ISegments.cs b/Source/StrongGrid/Resources/Legacy/ISegments.cs
index 1a9e1dd7..971755c4 100644
--- a/Source/StrongGrid/Resources/Legacy/ISegments.cs
+++ b/Source/StrongGrid/Resources/Legacy/ISegments.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -10,6 +11,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/campaigns.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ISegments
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/ISenderIdentities.cs b/Source/StrongGrid/Resources/Legacy/ISenderIdentities.cs
index e494cb2d..a3135de6 100644
--- a/Source/StrongGrid/Resources/Legacy/ISenderIdentities.cs
+++ b/Source/StrongGrid/Resources/Legacy/ISenderIdentities.cs
@@ -1,5 +1,6 @@
 using StrongGrid.Models;
 using StrongGrid.Utilities;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -11,6 +12,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/sender_identities.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public interface ISenderIdentities
 	{
 		/// <summary>
diff --git a/Source/StrongGrid/Resources/Legacy/Lists.cs b/Source/StrongGrid/Resources/Legacy/Lists.cs
index c4132e5a..f323ace2 100644
--- a/Source/StrongGrid/Resources/Legacy/Lists.cs
+++ b/Source/StrongGrid/Resources/Legacy/Lists.cs
@@ -15,6 +15,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/contactdb.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Lists : ILists
 	{
 		private const string _endpoint = "contactdb/lists";
diff --git a/Source/StrongGrid/Resources/Legacy/Segments.cs b/Source/StrongGrid/Resources/Legacy/Segments.cs
index e4689db9..95a7d41c 100644
--- a/Source/StrongGrid/Resources/Legacy/Segments.cs
+++ b/Source/StrongGrid/Resources/Legacy/Segments.cs
@@ -1,5 +1,6 @@
 using Pathoschild.Http.Client;
 using StrongGrid.Json;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
@@ -14,6 +15,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/campaigns.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class Segments : ISegments
 	{
 		private const string _endpoint = "contactdb/segments";
diff --git a/Source/StrongGrid/Resources/Legacy/SenderIdentities.cs b/Source/StrongGrid/Resources/Legacy/SenderIdentities.cs
index 1ba24321..ef3d9f6b 100644
--- a/Source/StrongGrid/Resources/Legacy/SenderIdentities.cs
+++ b/Source/StrongGrid/Resources/Legacy/SenderIdentities.cs
@@ -2,6 +2,7 @@
 using StrongGrid.Json;
 using StrongGrid.Models;
 using StrongGrid.Utilities;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -14,6 +15,7 @@ namespace StrongGrid.Resources.Legacy
 	/// <remarks>
 	/// See <a href="https://sendgrid.com/docs/API_Reference/Web_API_v3/Marketing_Campaigns/sender_identities.html">SendGrid documentation</a> for more information.
 	/// </remarks>
+	[Obsolete("The legacy client, legacy resources and legacy model classes are obsolete")]
 	public class SenderIdentities : ISenderIdentities
 	{
 		private const string _endpoint = "senders";