diff --git a/lib/rackstash/filters.rb b/lib/rackstash/filters.rb
index 7673d80..1963300 100644
--- a/lib/rackstash/filters.rb
+++ b/lib/rackstash/filters.rb
@@ -6,6 +6,8 @@
 # of the MIT license. See the LICENSE.txt file for details.
 
 require 'rackstash/filters/clear_color'
+require 'rackstash/filters/default_fields'
+require 'rackstash/filters/default_tags'
 require 'rackstash/filters/rename'
 require 'rackstash/filters/replace'
 require 'rackstash/filters/skip_event'
diff --git a/lib/rackstash/filters/default_fields.rb b/lib/rackstash/filters/default_fields.rb
new file mode 100644
index 0000000..630f514
--- /dev/null
+++ b/lib/rackstash/filters/default_fields.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+#
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+module Rackstash
+  module Filters
+    # The {DefaultFields} filter allows to define fields which should be added
+    # to an event if they aren't already explicitly set.
+    #
+    # Fields can be given either as a `Hash` or a `Proc` which in turn returns a
+    # `Hash` on `call`. The `Hash` can be nested arbitrarily deep.
+    #
+    # Each `Hash` value can again optionally be a `Proc` which is expected to
+    # return a field value on `call`. You can set nested Hashes or Arrays and
+    # define nested Procs which in turn are resolved recursively when applying
+    # the filter. That way, you can set lazy-evaluated values which are only
+    # resolved at the time the filter is applied to a logged event.
+    #
+    # @example
+    #   Rackstash::Flow.new(STDOUT) do
+    #     # All three defined filters set the same default fields
+    #     filter :default_fields, 'beep' => 'boop'
+    #     filter :default_fields, 'beep' => -> { 'boop' }
+    #     filter :default_fields, -> { { 'beep' => 'boop' } }
+    #   end
+    class DefaultFields
+      # @param default_fields [Hash<#to_s => Object>, Proc] a `Hash` specifying
+      #   default values for the named keys. You can either give a literal
+      #   `Hash` object or a `Proc` which returns such a `Hash`.
+      def initialize(default_fields)
+        @default_fields = default_fields
+      end
+
+      # Add the defined `default_fields` to the event hash, retaining all
+      # existing values.
+      #
+      # @param event [Hash] an event hash
+      # @return [Hash] the given `event` with the fields renamed
+      def call(event)
+        resolver = lambda do |key, old_val, new_val|
+          if old_val.nil?
+            new_val
+          elsif old_val.is_a?(Hash) && new_val.is_a?(Hash)
+            old_val.merge(new_val, &resolver)
+          elsif old_val.is_a?(Array) && new_val.is_a?(Array)
+            old_val | new_val
+          else
+            old_val
+          end
+        end
+
+        fields = Rackstash::Fields::Hash(@default_fields).to_h
+        event.merge!(fields, &resolver)
+      end
+    end
+  end
+end
diff --git a/lib/rackstash/filters/default_tags.rb b/lib/rackstash/filters/default_tags.rb
new file mode 100644
index 0000000..e117ad5
--- /dev/null
+++ b/lib/rackstash/filters/default_tags.rb
@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+#
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+module Rackstash
+  module Filters
+    # The {DefaultTags} filter allows to define tags which should be added
+    # to an event if they aren't already explicitly set there. All existing tags
+    # on the event are retained.
+    #
+    # The default tags are added to the `"tags"` field of the event `Hash`. They
+    # can be given either as an `Array` of `String`s or a `Proc` which in turn
+    # returns an `Array` of `String`s on `call`.
+    #
+    # Each value of the Array can again optionally be a Proc which in turn is
+    # expected to return a String on `call`. All the (potentially nested) procs
+    # are called recursively when applying the filter. That way, you can set
+    # lazy-evaluated values which are only resolved at the time the filter is
+    # applied to a logged event.
+    #
+    # @example
+    #   Rackstash::Flow.new(STDOUT) do
+    #     # All three defined filters set the same default tags
+    #     filter :default_tags, ['important', 'request']
+    #     filter :default_tags, -> { ['important', 'request'] }
+    #     filter :default_tags, ['important', -> { 'request' }]
+    #   end
+    class DefaultTags
+      # @param default_tags [Array<#to_s>, Set<#to_s>, Proc] an `Array`
+      #   specifying default tags for each event. You can either give a literal
+      #   `Array` containing Strings or a `Proc` which returns such an `Array`.
+      def initialize(*default_tags)
+        @default_tags = default_tags
+      end
+
+      # Add the defined `default_tags` to the event hash, retaining all
+      # existing tags. The `"tags"` field on the event will be normalized to a
+      # plain `Array` containing only `String`s.
+      #
+      # @param event [Hash] an event hash
+      # @return [Hash] the given `event` with the fields renamed
+      def call(event)
+        tags = Rackstash::Fields::Tags(event[FIELD_TAGS])
+        tags.merge!(@default_tags)
+
+        event[FIELD_TAGS] = tags.to_a
+        event
+      end
+    end
+  end
+end
diff --git a/spec/rackstash/filters/default_fields_spec.rb b/spec/rackstash/filters/default_fields_spec.rb
new file mode 100644
index 0000000..87cd93e
--- /dev/null
+++ b/spec/rackstash/filters/default_fields_spec.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+#
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/filters/default_fields'
+
+describe Rackstash::Filters::DefaultFields do
+  let(:event) {
+    {
+      'foo' => 'v1',
+      'bar' => 'v2'
+    }
+  }
+
+  def filter!(default_fields)
+    described_class.new(default_fields).call(event)
+  end
+
+  it 'adds missing normalized fields' do
+    filter! 'new' => 'boing', 123 => :number
+
+    expect(event).to eql({
+      'foo' => 'v1',
+      'bar' => 'v2',
+      'new' => 'boing',
+      '123' => 'number'
+    })
+  end
+
+  it 'retains existing fields' do
+    filter! foo: 'ignored'
+
+    expect(event).to eql({
+      'foo' => 'v1',
+      'bar' => 'v2'
+    })
+  end
+
+  it 'deep_merges fields' do
+    event['deep'] = { 'key' => [42, { 'foo' => 'bar' }, nil], 'new' => nil }
+    filter! 'deep' => { key: [123], new: 'new' }
+
+    expect(event).to eql({
+      'foo' => 'v1',
+      'bar' => 'v2',
+      'deep' => {
+        'key' => [42, { 'foo' => 'bar' }, nil, 123],
+        'new' => 'new'
+      }
+    })
+  end
+
+  it 'resolves Procs' do
+    filter! -> { { 'beep' => -> { 'boop' } } }
+
+    expect(event).to eql({
+      'foo' => 'v1',
+      'bar' => 'v2',
+      'beep' => 'boop'
+    })
+  end
+end
diff --git a/spec/rackstash/filters/default_tags_spec.rb b/spec/rackstash/filters/default_tags_spec.rb
new file mode 100644
index 0000000..40940fb
--- /dev/null
+++ b/spec/rackstash/filters/default_tags_spec.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+#
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/filters/default_tags'
+
+describe Rackstash::Filters::DefaultTags do
+  let(:event) {
+    {
+      'key' => 'value'
+    }
+  }
+
+  def filter!(*default_tags)
+    described_class.new(*default_tags).call(event)
+  end
+
+  it 'adds missing tags' do
+    filter! 'foo', 'bar'
+    expect(event['tags']).to eql ['foo', 'bar']
+  end
+
+  it 'retains existing tags' do
+    event['tags'] = ['tag']
+    filter! 'foo', 'bar'
+
+    expect(event['tags']).to eql ['tag', 'foo', 'bar']
+  end
+
+  it 'flattens and normalizes tags' do
+    event['tags'] = 'bare'
+    filter! [:foo, [[123]]]
+
+    expect(event['tags']).to eql ['bare', 'foo', '123']
+  end
+
+  it 'resolves Procs' do
+    filter! -> { ['beep', -> { ['boop'] }] }
+
+    expect(event['tags']).to eql ['beep', 'boop']
+  end
+end