diff --git a/src/common/mod.rs b/src/common/mod.rs
index 551840dc..b5d4bae1 100644
--- a/src/common/mod.rs
+++ b/src/common/mod.rs
@@ -57,6 +57,9 @@ pub use self::referer::Referer;
 pub use self::referrer_policy::ReferrerPolicy;
 pub use self::retry_after::RetryAfter;
 pub use self::sec_websocket_accept::SecWebsocketAccept;
+pub use self::sec_websocket_extensions::{
+    WebsocketExtensionParam, SecWebsocketExtensions, WebsocketProtocolExtension,
+};
 pub use self::sec_websocket_key::SecWebsocketKey;
 pub use self::sec_websocket_version::SecWebsocketVersion;
 pub use self::server::Server;
@@ -177,6 +180,7 @@ mod referer;
 mod referrer_policy;
 mod retry_after;
 mod sec_websocket_accept;
+mod sec_websocket_extensions;
 mod sec_websocket_key;
 mod sec_websocket_version;
 mod server;
diff --git a/src/common/sec_websocket_extensions.rs b/src/common/sec_websocket_extensions.rs
new file mode 100644
index 00000000..630701e5
--- /dev/null
+++ b/src/common/sec_websocket_extensions.rs
@@ -0,0 +1,338 @@
+use std::{borrow::Cow, fmt::Debug, iter::FromIterator, str::FromStr};
+
+use bytes::BytesMut;
+use headers_core::Error;
+use http::HeaderValue;
+
+use crate::util::{csv, TryFromValues};
+
+/// The `Sec-Websocket-Extensions` header.
+///
+/// This header is used in the Websocket handshake, sent by the client to the
+/// server and then from the server to the client. It is a proposed and
+/// agreed-upon list of websocket protocol extensions to use.
+#[derive(Clone, Debug, Default, PartialEq, Eq, Hash)]
+pub struct SecWebsocketExtensions(Vec<WebsocketProtocolExtension>);
+
+/// An extension listed in a [`SecWebsocketExtensions`] header.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct WebsocketProtocolExtension {
+    name: Cow<'static, str>,
+    params: Vec<WebsocketExtensionParam>,
+}
+
+/// Named parameter for an extension in a `Sec-Websocket-Extensions` header.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct WebsocketExtensionParam {
+    name: Cow<'static, str>,
+    value: Option<String>,
+}
+
+impl SecWebsocketExtensions {
+    /// Constructs a new header with the provided extensions.
+    pub fn new(extensions: impl IntoIterator<Item = WebsocketProtocolExtension>) -> Self {
+        Self(extensions.into_iter().collect())
+    }
+
+    /// Returns an iterator over the extensions in this header.
+    pub fn iter(&self) -> <&Self as IntoIterator>::IntoIter {
+        self.into_iter()
+    }
+}
+
+impl WebsocketProtocolExtension {
+    /// Constructs a new extension directive with the given name and parameters.
+    pub fn new(
+        name: impl Into<Cow<'static, str>>,
+        params: impl IntoIterator<Item = WebsocketExtensionParam>,
+    ) -> Self {
+        Self {
+            name: name.into(),
+            params: params.into_iter().collect(),
+        }
+    }
+
+    /// The name of this extension directive.
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+
+    /// Returns an iterator over the parameters for this extension directive.
+    pub fn params(&self) -> impl Iterator<Item = &WebsocketExtensionParam> {
+        self.params.iter()
+    }
+}
+
+impl WebsocketExtensionParam {
+    /// Constructs a new parameter with the given name and optional value.
+    #[inline]
+    pub fn new(name: impl Into<Cow<'static, str>>, value: Option<String>) -> Self {
+        Self {
+            name: name.into(),
+            value,
+        }
+    }
+
+    /// The name of the parameter.
+    #[inline]
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+
+    /// The parameter value, if there is one.
+    #[inline]
+    pub fn value(&self) -> Option<&str> {
+        self.value.as_deref()
+    }
+}
+
+impl crate::Header for SecWebsocketExtensions {
+    fn name() -> &'static ::http::header::HeaderName {
+        &::http::header::SEC_WEBSOCKET_EXTENSIONS
+    }
+
+    fn decode<'i, I>(values: &mut I) -> Result<Self, crate::Error>
+    where
+        I: Iterator<Item = &'i HeaderValue>,
+    {
+        crate::util::TryFromValues::try_from_values(values).map(SecWebsocketExtensions)
+    }
+    fn encode<E: Extend<crate::HeaderValue>>(&self, values: &mut E) {
+        values.extend(std::iter::once(to_header_value(&self.0)));
+    }
+}
+
+impl TryFromValues for Vec<WebsocketProtocolExtension> {
+    fn try_from_values<'i, I>(values: &mut I) -> Result<Self, Error>
+    where
+        Self: Sized,
+        I: Iterator<Item = &'i HeaderValue>,
+    {
+        csv::from_comma_delimited(values)
+    }
+}
+
+impl FromIterator<WebsocketProtocolExtension> for SecWebsocketExtensions {
+    fn from_iter<T: IntoIterator<Item = WebsocketProtocolExtension>>(iter: T) -> Self {
+        Self(iter.into_iter().collect())
+    }
+}
+
+impl IntoIterator for SecWebsocketExtensions {
+    type Item = WebsocketProtocolExtension;
+
+    type IntoIter = std::vec::IntoIter<Self::Item>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a SecWebsocketExtensions {
+    type Item = &'a WebsocketProtocolExtension;
+
+    type IntoIter = std::slice::Iter<'a, WebsocketProtocolExtension>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.iter()
+    }
+}
+
+impl FromStr for WebsocketProtocolExtension {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (name, tail) = s
+            .split_once(';')
+            .map(|(n, t)| (n, Some(t)))
+            .unwrap_or((s, None));
+
+        let params = csv::from_delimited(&mut tail.into_iter(), ';')?;
+
+        Ok(Self {
+            name: name.trim().to_owned().into(),
+            params,
+        })
+    }
+}
+
+impl std::fmt::Display for WebsocketProtocolExtension {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let Self { name, params } = self;
+
+        write!(f, "{}", name)?;
+        for param in params {
+            f.write_str("; ")?;
+            write!(f, "{}", param)?;
+        }
+
+        Ok(())
+    }
+}
+
+impl FromStr for WebsocketExtensionParam {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (name, value) = s
+            .split_once('=')
+            .map(|(n, t)| (n, Some(t)))
+            .unwrap_or((s, None));
+
+        let value = value.map(|value| value.trim().to_owned());
+
+        Ok(Self {
+            name: name.trim().to_owned().into(),
+            value,
+        })
+    }
+}
+
+impl std::fmt::Display for WebsocketExtensionParam {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let Self { name, value } = self;
+
+        write!(f, "{}", name)?;
+        if let Some(value) = value {
+            write!(f, "={}", value)?;
+        }
+        Ok(())
+    }
+}
+
+impl WebsocketProtocolExtension {
+    fn encoded_len(&self) -> usize {
+        let Self { name, params } = self;
+
+        let params_len: usize = params.iter().map(|p| p.encoded_len() + 2).sum();
+
+        name.len() + params_len
+    }
+}
+
+impl WebsocketExtensionParam {
+    fn encoded_len(&self) -> usize {
+        let Self { name, value } = self;
+        name.len() + value.as_ref().map(|s| s.len() + 1).unwrap_or_default()
+    }
+
+    fn write_to_buffer(&self, buffer: &mut BytesMut) {
+        let Self { name, value } = self;
+        buffer.extend_from_slice(b"; ");
+        buffer.extend_from_slice(name.as_bytes());
+
+        if let Some(value) = value {
+            buffer.extend_from_slice(b"=");
+            buffer.extend_from_slice(value.as_bytes());
+        }
+    }
+}
+
+fn to_header_value(extensions: &[WebsocketProtocolExtension]) -> HeaderValue {
+    let mut buffer = BytesMut::with_capacity(encoded_len(extensions));
+
+    for extension in extensions {
+        if !buffer.is_empty() {
+            buffer.extend_from_slice(b", ");
+        }
+
+        let WebsocketProtocolExtension { name, params } = extension;
+        buffer.extend_from_slice(name.as_bytes());
+
+        for param in params {
+            param.write_to_buffer(&mut buffer);
+        }
+    }
+
+    HeaderValue::from_maybe_shared(buffer).expect("valid construction")
+}
+
+fn encoded_len(extensions: &[WebsocketProtocolExtension]) -> usize {
+    let all_encoded_len: usize = extensions
+        .iter()
+        .map(WebsocketProtocolExtension::encoded_len)
+        .sum();
+    let all_separators_len = extensions
+        .len()
+        .checked_sub(1)
+        .map(|num_separators| num_separators * 2)
+        .unwrap_or_default();
+    all_encoded_len + all_separators_len
+}
+
+#[cfg(test)]
+mod tests {
+    use std::convert::TryInto;
+
+    use crate::Header;
+
+    use super::super::{test_decode, test_encode};
+    use super::*;
+
+    #[test]
+    fn parse_separate_headers() {
+        // From https://tools.ietf.org/html/rfc6455#section-9.1
+        let extensions =
+            test_decode::<SecWebsocketExtensions>(&["foo", "bar; baz=2"]).expect("valid");
+
+        assert_eq!(
+            extensions,
+            SecWebsocketExtensions(vec![
+                WebsocketProtocolExtension {
+                    name: "foo".into(),
+                    params: vec![],
+                },
+                WebsocketProtocolExtension {
+                    name: "bar".into(),
+                    params: vec![WebsocketExtensionParam {
+                        name: "baz".into(),
+                        value: Some("2".to_owned())
+                    }],
+                }
+            ])
+        );
+    }
+
+    #[test]
+    fn round_trip_complex() {
+        let extensions = test_decode::<SecWebsocketExtensions>(&[
+            "deflate-stream",
+            "mux; max-channels=4; flow-control, deflate-stream",
+            "private-extension",
+        ])
+        .expect("valid");
+
+        let headers = test_encode(extensions);
+        assert_eq!(
+            headers["sec-websocket-extensions"],
+            "deflate-stream, mux; max-channels=4; flow-control, deflate-stream, private-extension"
+        );
+    }
+
+    #[test]
+    fn to_header_value_exact() {
+        // This isn't a required property for correctness but if the length
+        // precomputation is wrong we'll over- or under-allocate during
+        // conversion.
+        let cases = [
+            SecWebsocketExtensions::new([
+                WebsocketProtocolExtension::from_str("extension-name").unwrap(),
+                WebsocketProtocolExtension::from_str("with-params; a=5; b=8").unwrap(),
+            ]),
+            SecWebsocketExtensions::new([]),
+            SecWebsocketExtensions::new([
+                WebsocketProtocolExtension::from_str("duplicate-name").unwrap(),
+                WebsocketProtocolExtension::from_str("duplicate-name").unwrap(),
+                WebsocketProtocolExtension::from_str("duplicate-name").unwrap(),
+            ]),
+        ];
+
+        for case in cases {
+            let mut values = Vec::new();
+            case.encode(&mut values);
+            let [value] = values.try_into().unwrap();
+
+            assert_eq!(value.len(), encoded_len(&case.0));
+        }
+    }
+}
diff --git a/src/util/csv.rs b/src/util/csv.rs
index a3d05ba4..32c1737b 100644
--- a/src/util/csv.rs
+++ b/src/util/csv.rs
@@ -10,36 +10,47 @@ where
     I: Iterator<Item = &'i HeaderValue>,
     T: ::std::str::FromStr,
     E: ::std::iter::FromIterator<T>,
+{
+    from_delimited(
+        &mut values.flat_map(|header_value| header_value.to_str()),
+        ',',
+    )
+}
+
+/// Reads a single-character-delimited raw header into a Vec.
+pub(crate) fn from_delimited<'i, I, T, E>(values: &mut I, delimiter: char) -> Result<E, Error>
+where
+    I: Iterator<Item = &'i str>,
+    T: ::std::str::FromStr,
+    E: ::std::iter::FromIterator<T>,
 {
     values
-        .flat_map(|value| {
-            value.to_str().into_iter().flat_map(|string| {
-                let mut in_quotes = false;
-                string
-                    .split(move |c| {
-                        #[allow(clippy::collapsible_else_if)]
-                        if in_quotes {
+        .flat_map(|string| {
+            let mut in_quotes = false;
+            string
+                .split(move |c| {
+                    #[allow(clippy::collapsible_else_if)]
+                    if in_quotes {
+                        if c == '"' {
+                            in_quotes = false;
+                        }
+                        false // dont split
+                    } else {
+                        if c == delimiter {
+                            true // split
+                        } else {
                             if c == '"' {
-                                in_quotes = false;
+                                in_quotes = true;
                             }
                             false // dont split
-                        } else {
-                            if c == ',' {
-                                true // split
-                            } else {
-                                if c == '"' {
-                                    in_quotes = true;
-                                }
-                                false // dont split
-                            }
                         }
-                    })
-                    .filter_map(|x| match x.trim() {
-                        "" => None,
-                        y => Some(y),
-                    })
-                    .map(|x| x.parse().map_err(|_| Error::invalid()))
-            })
+                    }
+                })
+                .filter_map(|x| match x.trim() {
+                    "" => None,
+                    y => Some(y),
+                })
+                .map(|x| x.parse().map_err(|_| Error::invalid()))
         })
         .collect()
 }