diff --git a/README.md b/README.md
index bcfd5a2..d41b443 100644
--- a/README.md
+++ b/README.md
@@ -95,7 +95,29 @@ $customerio.identify(
 )
 ```
 
-### Updating customers
+### Updating customers: Using other IDs
+
+If you wish to specify a customer ID that is different than the one used in the `id` attribute, you can do so by using the `identify_customer_id` method:
+
+```ruby
+# Arguments
+# customer_id (required) - the customer ID to use for this customer, may be an id, email address, or the cio_id.
+#                         This will be used to construct the URL but not sent in the body attributes.
+# attributes (required) - a hash of information about the customer. You can pass any
+#                         information that would be useful in your triggers. You
+#                         must at least pass in an id, email, and created_at timestamp.
+
+$customerio.identify_customer_id(
+  :customer_id => "bob@example.com",
+  :id => 5,
+  :email => "bob@example.com",
+  :created_at => customer.created_at.to_i,
+  :first_name => "Bob",
+  :plan => "basic"
+)
+```
+
+### Updating customers: Changing identifiers
 
 You can use the identify operation to update customers.
 If you need to change the `id` or `email` identifiers for a customer,
diff --git a/lib/customerio/client.rb b/lib/customerio/client.rb
index e49a6cc..f3a5dcd 100644
--- a/lib/customerio/client.rb
+++ b/lib/customerio/client.rb
@@ -29,6 +29,10 @@ def identify(attributes)
       create_or_update(attributes)
     end
 
+    def identify_customer_id(customer_id: nil, **attributes)
+      create_or_update_customer_id(customer_id, **attributes)
+    end
+
     def delete(customer_id)
       raise ParamError.new("customer_id must be a non-empty string") if is_empty?(customer_id)
       @client.request_and_verify_response(:delete, customer_path(customer_id))
@@ -161,6 +165,14 @@ def create_or_update(attributes = {})
       if !is_empty?(attributes[:cio_id])
         customer_id = "cio_" + attributes[:cio_id]
       end
+      create_or_update_customer_id(customer_id, attributes)
+    end
+
+    def create_or_update_customer_id(customer_id, attributes = {})
+      attributes = Hash[attributes.map { |(k,v)| [ k.to_sym, v ] }]
+      if is_empty?(customer_id)
+        raise MissingIdAttributeError.new("Must provide a customer id")
+      end
       url = customer_path(customer_id)
       @client.request_and_verify_response(:put, url, attributes)
     end
diff --git a/spec/client_spec.rb b/spec/client_spec.rb
index 41df3cd..b59f9a0 100644
--- a/spec/client_spec.rb
+++ b/spec/client_spec.rb
@@ -225,6 +225,50 @@ def json(data)
     end
   end
 
+  describe "#identify_customer_id" do
+    it "uses provided id rather than id" do
+      stub_request(:put, api_uri('/api/v1/customers/1234')).
+        with(body: json(id: "5")).
+        to_return(status: 200, body: "", headers: {})
+
+      client.identify_customer_id(
+        customer_id: "1234",
+        id: "5"
+      )
+    end
+
+    it "uses provided cio_id rather than id" do
+      stub_request(:put, api_uri('/api/v1/customers/cio_5')).
+        with(body: json(id: "5")).
+        to_return(status: 200, body: "", headers: {})
+
+      client.identify_customer_id(
+        customer_id: "cio_5",
+        id: "5"
+      )
+    end
+
+    it "uses provided email rather than id" do
+      stub_request(:put, api_uri('/api/v1/customers/customer@example.com')).
+        with(body: json(id: "5")).
+        to_return(status: 200, body: "", headers: {})
+
+      client.identify_customer_id(
+        customer_id: "customer@example.com",
+        id: "5"
+      )
+    end
+
+    it "requires a customer_id attribute" do
+      lambda { client.identify_customer_id() }.should raise_error(Customerio::Client::MissingIdAttributeError)
+      lambda { client.identify_customer_id(customer_id: "") }.should raise_error(Customerio::Client::MissingIdAttributeError)
+      # None of these are customer_id
+      lambda { client.identify_customer_id(id: "5") }.should raise_error(Customerio::Client::MissingIdAttributeError)
+      lambda { client.identify_customer_id(cio_id: "cio_5") }.should raise_error(Customerio::Client::MissingIdAttributeError)
+      lambda { client.identify_customer_id(email: "customer@example.com") }.should raise_error(Customerio::Client::MissingIdAttributeError)
+    end
+  end
+
   describe "#delete" do
     it "sends a DELETE request to the customer.io's event API" do
       stub_request(:delete, api_uri('/api/v1/customers/5')).
@@ -650,7 +694,7 @@ def json(data)
       }.to raise_error(Customerio::Client::ParamError, 'timestamp must be a valid timestamp')
     end
   end
-  
+
   describe "#merge_customers" do
     before(:each) do
       @client = Customerio::Client.new("SITE_ID", "API_KEY", :json => true)