diff --git a/contract-tests/client_entity.rb b/contract-tests/client_entity.rb
index dc2043d4..99a177ba 100644
--- a/contract-tests/client_entity.rb
+++ b/contract-tests/client_entity.rb
@@ -15,11 +15,13 @@ def initialize(log, config)
     if config[:streaming]
       streaming = config[:streaming]
       opts[:stream_uri] = streaming[:baseUri] unless streaming[:baseUri].nil?
+      opts[:payload_filter_key] = streaming[:filter] unless streaming[:filter].nil?
       opts[:initial_reconnect_delay] = streaming[:initialRetryDelayMs] / 1_000.0 unless streaming[:initialRetryDelayMs].nil?
     elsif config[:polling]
       polling = config[:polling]
       opts[:stream] = false
       opts[:base_uri] = polling[:baseUri] unless polling[:baseUri].nil?
+      opts[:payload_filter_key] = polling[:filter] unless polling[:filter].nil?
       opts[:poll_interval] = polling[:pollIntervalMs] / 1_000.0 unless polling[:pollIntervalMs].nil?
     end
 
diff --git a/contract-tests/service.rb b/contract-tests/service.rb
index c542f6a2..853a63c1 100644
--- a/contract-tests/service.rb
+++ b/contract-tests/service.rb
@@ -30,6 +30,7 @@
       'all-flags-with-reasons',
       'all-flags-client-side-only',
       'all-flags-details-only-for-tracked-flags',
+      'filtering',
       'secure-mode-hash',
       'user-type',
       'tags',
diff --git a/lib/ldclient-rb/config.rb b/lib/ldclient-rb/config.rb
index 498d33d6..536e9019 100644
--- a/lib/ldclient-rb/config.rb
+++ b/lib/ldclient-rb/config.rb
@@ -57,6 +57,7 @@ class Config
     # @option opts [#open] :socket_factory See {#socket_factory}.
     # @option opts [BigSegmentsConfig] :big_segments See {#big_segments}.
     # @option opts [Hash] :application See {#application}
+    # @option opts [String] :payload_filter_key See {#payload_filter_key}
     #
     def initialize(opts = {})
       @base_uri = (opts[:base_uri] || Config.default_base_uri).chomp("/")
@@ -88,6 +89,7 @@ def initialize(opts = {})
       @socket_factory = opts[:socket_factory]
       @big_segments = opts[:big_segments] || BigSegmentsConfig.new(store: nil)
       @application = LaunchDarkly::Impl::Util.validate_application_info(opts[:application] || {}, @logger)
+      @payload_filter_key = opts[:payload_filter_key]
     end
 
     #
@@ -330,6 +332,21 @@ def offline?
     #
     attr_reader :application
 
+    #
+    # LaunchDarkly Server SDKs historically downloaded all flag configuration and segments for a particular environment
+    # during initialization.
+    #
+    # For some customers, this is an unacceptably large amount of data, and has contributed to performance issues within
+    # their products.
+    #
+    # Filtered environments aim to solve this problem. By allowing customers to specify subsets of an environment's
+    # flags using a filter key, SDKs will initialize faster and use less memory.
+    #
+    # This payload filter key only applies to the default streaming and polling data sources. It will not affect TestData or FileData
+    # data sources, nor will it be applied to any data source provided through the {#data_source} config property.
+    #
+    attr_reader :payload_filter_key
+
     #
     # Set to true to opt out of sending diagnostics data.
     #
diff --git a/lib/ldclient-rb/requestor.rb b/lib/ldclient-rb/requestor.rb
index 58db38ab..c58ec81a 100644
--- a/lib/ldclient-rb/requestor.rb
+++ b/lib/ldclient-rb/requestor.rb
@@ -43,12 +43,10 @@ def stop
 
     private
 
-    def request_single_item(kind, path)
-      Impl::Model.deserialize(kind, make_request(path), @config.logger)
-    end
-
     def make_request(path)
-      uri = URI(@config.base_uri + path)
+      uri = URI(
+        Util.add_payload_filter_key(@config.base_uri + path, @config)
+      )
       headers = {}
       Impl::Util.default_http_headers(@sdk_key, @config).each { |k, v| headers[k] = v }
       headers["Connection"] = "keep-alive"
diff --git a/lib/ldclient-rb/stream.rb b/lib/ldclient-rb/stream.rb
index d6ab5086..6d8dd1bb 100644
--- a/lib/ldclient-rb/stream.rb
+++ b/lib/ldclient-rb/stream.rb
@@ -51,7 +51,9 @@ def start
         reconnect_time: @config.initial_reconnect_delay,
       }
       log_connection_started
-      @es = SSE::Client.new(@config.stream_uri + "/all", **opts) do |conn|
+
+      uri = Util.add_payload_filter_key(@config.stream_uri + "/all", @config)
+      @es = SSE::Client.new(uri, **opts) do |conn|
         conn.on_event { |event| process_message(event) }
         conn.on_error { |err|
           log_connection_result(false)
diff --git a/lib/ldclient-rb/util.rb b/lib/ldclient-rb/util.rb
index df4ae191..5fcc23d4 100644
--- a/lib/ldclient-rb/util.rb
+++ b/lib/ldclient-rb/util.rb
@@ -4,6 +4,32 @@
 module LaunchDarkly
   # @private
   module Util
+    #
+    # Append the payload filter key query parameter to the provided URI.
+    #
+    # @param uri [String]
+    # @param config [Config]
+    # @return [String]
+    #
+    def self.add_payload_filter_key(uri, config)
+      return uri if config.payload_filter_key.nil?
+
+      unless config.payload_filter_key.is_a?(String) && !config.payload_filter_key.empty?
+        config.logger.warn { "[LDClient] Filter key must be a non-empty string. No filtering will be applied." }
+        return uri
+      end
+
+      begin
+        parsed = URI.parse(uri)
+        new_query_params = URI.decode_www_form(String(parsed.query)) << ["filter", config.payload_filter_key]
+        parsed.query = URI.encode_www_form(new_query_params)
+        parsed.to_s
+      rescue URI::InvalidURIError
+        config.logger.warn { "[LDClient] URI could not be parsed. No filtering will be applied." }
+        uri
+      end
+    end
+
     def self.new_http_client(uri_s, config)
       http_client_options = {}
       if config.socket_factory