diff --git a/NEWS b/NEWS
index b43190a..b9244e1 100644
--- a/NEWS
+++ b/NEWS
@@ -327,6 +327,11 @@ with all sufficient information, see the ChangeLog file.
     * Readline.completion_proc= accepts nil.
       nil means to use default completion proc.
 
+* set
+  * new methods:
+    * Set#keep_if
+    * Set#select!
+
 * time
   * incompatible changes:
     * Time.parse raises ArgumentError when no date information.
diff --git a/lib/set.rb b/lib/set.rb
index 0835f8a..93db118 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -266,6 +266,14 @@ class Set
     self
   end
 
+  # Deletes every element of the set for which block evaluates to
+  # false, and returns self.
+  def keep_if
+    block_given? or return enum_for(__method__)
+    to_a.each { |o| @hash.delete(o) unless yield(o) }
+    self
+  end
+
   # Replaces the elements with ones returned by collect().
   def collect!
     block_given? or return enum_for(__method__)
@@ -284,6 +292,15 @@ class Set
     size == n ? nil : self
   end
 
+  # Equivalent to Set#keep_if, but returns nil if no changes were
+  # made.
+  def select!
+    block_given? or return enum_for(__method__)
+    n = size
+    keep_if { |o| yield(o) }
+    size == n ? nil : self
+  end
+
   # Merges the elements of the given enumerable object to the set and
   # returns self.
   def merge(enum)