diff --git a/shard.yml b/shard.yml
index cf78d70..19be6f5 100644
--- a/shard.yml
+++ b/shard.yml
@@ -1,5 +1,5 @@
 name: optarg
-version: 0.5.4
+version: 0.5.5
 
 authors:
   - mosop
diff --git a/spec/internal/unknown_spec.cr b/spec/internal/unknown_spec.cr
new file mode 100644
index 0000000..58eda10
--- /dev/null
+++ b/spec/internal/unknown_spec.cr
@@ -0,0 +1,26 @@
+require "../spec_helper"
+
+module OptargInternalSpec::Unknown
+  class Model < Optarg::Model
+    string "-s"
+    unknown
+  end
+
+  describe name do
+    it "with option" do
+      result = Model.parse(%w(-s foo bar))
+      result.s?.should eq "foo"
+      result.unparsed_args.should eq %w(bar)
+    end
+
+    it "without different option" do
+      result = Model.parse(%w(-b foo bar))
+      result.unparsed_args.should eq %w(-b foo bar)
+    end
+
+    it "without option" do
+      result = Model.parse(%w(foo bar))
+      result.unparsed_args.should eq %w(foo bar)
+    end
+  end
+end
diff --git a/src/lib/completion_generators/base.cr b/src/lib/completion_generators/base.cr
index 4d84d22..2196c4f 100644
--- a/src/lib/completion_generators/base.cr
+++ b/src/lib/completion_generators/base.cr
@@ -100,6 +100,7 @@ module Optarg::CompletionGenerators
     end
 
     def make_definition(df)
+      return if df.is_a?(Definitions::Unknown)
       @keymap[df.key] = @keymap.size
       add_key df
       add_act df
diff --git a/src/lib/definition_set.cr b/src/lib/definition_set.cr
index 7e679f7..1aa25bd 100644
--- a/src/lib/definition_set.cr
+++ b/src/lib/definition_set.cr
@@ -36,6 +36,7 @@ module Optarg
       __set Definitions::Terminator, terminators
       __set DefinitionMixins::Value, values
       __set Definitions::StringArrayArgument, string_array_arguments
+      __set Definitions::Unknown, unknowns
     end
 
     getter all = {} of String => Definitions::Base
@@ -44,6 +45,7 @@ module Optarg
     getter value_options = {} of String => DefinitionMixins::ValueOption
     getter handlers = {} of String => Definitions::Handler
     getter terminators = {} of String => Definitions::Terminator
+    getter unknowns = {} of String => Definitions::Unknown
     getter values = {} of String => DefinitionMixins::Value
     getter string_array_arguments = {} of String => Definitions::StringArrayArgument
 
diff --git a/src/lib/definitions/base.cr b/src/lib/definitions/base.cr
index 95f7b9d..31686fb 100644
--- a/src/lib/definitions/base.cr
+++ b/src/lib/definitions/base.cr
@@ -7,7 +7,7 @@ module Optarg::Definitions
     getter names : Array(String)
     getter metadata : Metadata
 
-    def initialize(names : String | Array(String), metadata : Metadata? = nil, stop : Bool? = nil, terminate : Bool? = nil, complete : String | Symbol | Array(String) | Nil = nil)
+    def initialize(names : String | Array(String), metadata : Metadata? = nil, stop : Bool? = nil, terminate : Bool? = nil, complete : String | Symbol | Array(String) | Nil = nil, unknown : Bool? = nil)
       @names = case names
       when String
         [names]
@@ -18,6 +18,7 @@ module Optarg::Definitions
       @metadata.definition = self
       @stops = !!stop
       @terminates = !!terminate
+      @unknown = !!unknown
       initialize_completion complete: (complete || "")
     end
 
@@ -31,6 +32,11 @@ module Optarg::Definitions
       @terminates.as(Bool)
     end
 
+    @unknown : Bool?
+    def unknown?
+      @unknown.as(Bool)
+    end
+
     def key
       @names[0]
     end
diff --git a/src/lib/definitions/unknown.cr b/src/lib/definitions/unknown.cr
new file mode 100644
index 0000000..6307122
--- /dev/null
+++ b/src/lib/definitions/unknown.cr
@@ -0,0 +1,15 @@
+module Optarg::Definitions
+  class Unknown < Base
+    def initialize(metadata = nil)
+      super "@unknown", metadata: metadata, unknown: true
+    end
+
+    def completion_length(gen)
+      1
+    end
+
+    def completion_max_occurs(gen)
+      1
+    end
+  end
+end
diff --git a/src/lib/model/dsl/unknown.cr b/src/lib/model/dsl/unknown.cr
new file mode 100644
index 0000000..efd7203
--- /dev/null
+++ b/src/lib/model/dsl/unknown.cr
@@ -0,0 +1,9 @@
+module Optarg
+  class Model
+    # Defines an unknown model item.
+    macro unknown(metadata = nil)
+      %unknown = ::Optarg::Definitions::Unknown.new(metadata: {{metadata}})
+      @@__klass.definitions << %unknown
+    end
+  end
+end
diff --git a/src/lib/parser.cr b/src/lib/parser.cr
index f62dc35..549bbfa 100644
--- a/src/lib/parser.cr
+++ b/src/lib/parser.cr
@@ -60,7 +60,7 @@ module Optarg
     # :nodoc:
     def stopped?
       return false if parsed_nodes.size == 0
-      return parsed_nodes.last[:definitions].any?{|i| i.stops? || i.terminates?}
+      return parsed_nodes.last[:definitions].any?{|i| i.stops? || i.terminates? || i.unknown?}
     end
 
     on_validate do |o|
@@ -101,13 +101,21 @@ module Optarg
     def visit
       arg = self[0]
       if visit_terminator
-      elsif arg =~ /^-\w\w/
+        return
+      end
+      if arg =~ /^-\w\w/
         visit_concatenated_options
-      elsif arg =~ /^-/
-        visit_option
-      else
-        visit_argument
+        return
       end
+      if arg =~ /^-/
+        begin
+          visit_option
+          return
+        rescue ex : UnknownOption
+          raise ex if definitions.unknowns.empty?
+        end
+      end
+      visit_argument
     end
 
     # :nodoc:
@@ -157,6 +165,15 @@ module Optarg
 
     # :nodoc:
     def visit_argument
+      if definitions.unknowns.empty?
+        visit_argument2
+      else
+        visit_unknown
+      end
+    end
+
+    # :nodoc:
+    def visit_argument2
       while @argument_index < definitions.arguments.size
         df = definitions.argument_list[@argument_index]
         unless df.visitable?(self)
@@ -175,6 +192,12 @@ module Optarg
       @index += 1
     end
 
+    # :nodoc:
+    def visit_unknown
+      node = Parser.new_node(%w(), definitions.unknowns.first[1])
+      @parsed_nodes << node
+    end
+
     # :nodoc:
     def [](*args)
       input_args[@index..-1][*args]
diff --git a/src/version.cr b/src/version.cr
index 6c72d14..51c3010 100644
--- a/src/version.cr
+++ b/src/version.cr
@@ -1,3 +1,3 @@
 module Optarg
-  VERSION = "0.5.4"
+  VERSION = "0.5.5"
 end