diff --git a/build_tests/variants/default.rb b/build_tests/variants/default.rb
new file mode 100644
index 0000000..44ddae6
--- /dev/null
+++ b/build_tests/variants/default.rb
@@ -0,0 +1,8 @@
+variant "debug", default: false
+variant "release"
+
+with_variants do
+  env "prog" do |env|
+    env.Program("^/prog.exe", "prog.c")
+  end
+end
diff --git a/build_tests/variants/error_with_variants_with_empty_variant_group.rb b/build_tests/variants/error_with_variants_with_empty_variant_group.rb
deleted file mode 100644
index 8a65075..0000000
--- a/build_tests/variants/error_with_variants_with_empty_variant_group.rb
+++ /dev/null
@@ -1,5 +0,0 @@
-variant_group
-variant_group
-variant "foo"
-with_variants do
-end
diff --git a/lib/rscons/application.rb b/lib/rscons/application.rb
index 4afa5e0..9b3d817 100644
--- a/lib/rscons/application.rb
+++ b/lib/rscons/application.rb
@@ -5,8 +5,8 @@ module Rscons
   # Functionality for an instance of the rscons application invocation.
   class Application
 
-    # @return [Array<String>]
-    #   Active variant names.
+    # @return [Array<Hash>]
+    #   Active variants.
     attr_reader :active_variants
 
     # @return [String]
@@ -54,11 +54,19 @@ module Rscons
     #   List of task(s) to execute.
     # @param show_tasks [Boolean]
     #   Flag to show tasks and exit.
+    # @param enabled_variants [String]
+    #   User-specified variants list.
     #
     # @return [Integer]
     #   Process exit code (0 on success).
-    def run(rsconscript, tasks_and_params, show_tasks)
+    def run(rsconscript, tasks_and_params, show_tasks, enabled_variants)
       Cache.instance["failed_commands"] = []
+      @enabled_variants = enabled_variants
+      if enabled_variants == "" && !tasks_and_params.include?("configure")
+        if cache_enabled_variants = Cache.instance["configuration_data"]["enabled_variants"]
+          @enabled_variants = cache_enabled_variants
+        end
+      end
       @script = Script.new
       @script.load(rsconscript)
       if show_tasks
@@ -66,6 +74,7 @@ module Rscons
         return 0
       end
       apply_task_params(tasks_and_params)
+      enable_variants
       if tasks_and_params.empty?
         check_process_environments
         if Task.tasks["default"]
@@ -79,6 +88,32 @@ module Rscons
       0
     end
 
+    # Apply user-specified variant enables and complain if they don't make
+    # sense given the build script variant configuration.
+    def enable_variants
+      unless @_variants_enabled
+        if @enabled_variants != ""
+          exact = !(@enabled_variants =~ /^(\+|-)/)
+          enabled_variants = @enabled_variants.split(",")
+          specified_variants = {}
+          enabled_variants.each do |enable_variant|
+            enable_variant =~ /^(\+|-)?(.*)$/
+            enable_disable, variant_name = $1, $2
+            specified_variants[variant_name] = enable_disable != "-"
+          end
+          each_variant do |variant|
+            if specified_variants.include?(variant[:name])
+              variant[:enabled] = specified_variants[variant[:name]]
+            elsif exact
+              variant[:enabled] = false
+            end
+          end
+        end
+        @_variants_enabled = true
+      end
+      check_enabled_variants
+    end
+
     # Show the last failures.
     #
     # @return [void]
@@ -131,6 +166,7 @@ module Rscons
     #
     # @return [void]
     def check_configure
+      enable_variants
       unless Cache.instance["configuration_data"]["configured"]
         if @script.autoconf
           configure
@@ -168,6 +204,7 @@ module Rscons
         co.close(false)
         raise e
       end
+      Cache.instance["configuration_data"]["enabled_variants"] = @enabled_variants
       co.close(true)
     end
 
@@ -213,24 +250,48 @@ module Rscons
         end
         options = options.dup
         options[:name] = name
-        options[:active] = options.fetch(:default, true)
+        options[:enabled] = options.fetch(:default, true)
         options[:key] = options.fetch(:key, name)
         @variant_groups.last[:variants] << options
       end
     end
 
+    # Check if a variant is enabled.
+    #
+    # This can be used, for example, in a configuration block to omit or
+    # include configuration checks based on which variants have been
+    # configured.
+    #
+    # @param variant_name [String]
+    #   Variant name.
+    #
+    # @return [Boolean]
+    #   Whether the requested variant is enabled.
+    def variant_enabled?(variant_name)
+      each_variant do |variant|
+        if variant[:name] == variant_name
+          return variant[:enabled]
+        end
+      end
+      false
+    end
+
     # Create a variant group.
     def variant_group(*args, &block)
       if args.first.is_a?(String)
         name = args.slice!(0)
       end
-      @variant_groups << {name: name, variants: []}
+      options = args.first || {}
+      @variant_groups << options.merge(name: name, variants: [])
       if block
         block[]
       end
     end
 
-    # Iterate through variants.
+    # Iterate through enabled variants.
+    #
+    # The given block is called for each combination of enabled variants
+    # across the defined variant groups.
     def with_variants(&block)
       if @active_variants
         raise "with_variants cannot be called within another with_variants block"
@@ -238,18 +299,15 @@ module Rscons
       if @variant_groups.empty?
         raise "with_variants cannot be called with no variants defined"
       end
-      if @variant_groups.any? {|variant_group| variant_group[:variants].empty?}
-        raise "Error: empty variant group found"
-      end
-      iter_vgs = lambda do |variants|
-        if variants.size == @variant_groups.size
-          @active_variants = variants
+      iter_vgs = lambda do |iter_variants|
+        if iter_variants.size == @variant_groups.size
+          @active_variants = iter_variants.compact
           block[]
           @active_variants = nil
         else
-          @variant_groups[variants.size][:variants].each do |variant|
-            if variant[:active]
-              iter_vgs[variants + [variant]]
+          @variant_groups[iter_variants.size][:variants].each do |variant|
+            if variant[:enabled]
+              iter_vgs[iter_variants + [variant]]
             end
           end
         end
@@ -259,6 +317,29 @@ module Rscons
 
     private
 
+    def check_enabled_variants
+      @variant_groups.each do |variant_group|
+        enabled_count = variant_group[:variants].count do |variant|
+          variant[:enabled]
+        end
+        if enabled_count == 0
+          message = "No variants enabled for variant group"
+          if variant_group[:name]
+            message += " #{variant_group[:name].inspect}"
+          end
+          raise RsconsError.new(message)
+        end
+      end
+    end
+
+    def each_variant
+      @variant_groups.each do |variant_group|
+        variant_group[:variants].each do |variant|
+          yield variant
+        end
+      end
+    end
+
     def show_script_tasks
       puts "Tasks:"
       Task[].sort.each do |task_name, task|
diff --git a/lib/rscons/cli.rb b/lib/rscons/cli.rb
index 2af0580..9f60792 100644
--- a/lib/rscons/cli.rb
+++ b/lib/rscons/cli.rb
@@ -54,6 +54,7 @@ module Rscons
     def run_toplevel(argv)
       rsconscript = nil
       show_tasks = false
+      enabled_variants = ""
 
       OptionParser.new do |opts|
 
@@ -61,6 +62,10 @@ module Rscons
           Rscons.application.build_dir = build_dir
         end
 
+        opts.on("-e", "--variants VS") do |variants|
+          enabled_variants = variants
+        end
+
         opts.on("-f FILE") do |f|
           rsconscript = f
         end
@@ -127,7 +132,7 @@ module Rscons
       end
 
       begin
-        Rscons.application.run(rsconscript, tasks_and_params, show_tasks)
+        Rscons.application.run(rsconscript, tasks_and_params, show_tasks, enabled_variants)
       rescue RsconsError => e
         Ansi.write($stderr, :red, e.message, :reset, "\n")
         1
@@ -140,6 +145,7 @@ Usage: #{$0} [global options] [[task] [task options] ...]
 
 Global options:
   -b BUILD, --build=BUILD     Set build directory (default: build)
+  -e VS, --variants=VS        Enable or disable variants
   -f FILE                     Use FILE as Rsconscript
   -F, --show-failure          Show failed command log from previous build and exit
   -h, --help                  Show rscons help and exit
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index 08f62fc..9eaf5f1 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -102,7 +102,6 @@ module Rscons
       end.compact
       @name = [base_name, *variant_keys].join("-")
       options = args.first || {}
-      Rscons.application.check_configure
       unless Cache.instance["configuration_data"]["configured"]
         raise "Project must be configured before creating an Environment"
       end
diff --git a/lib/rscons/script.rb b/lib/rscons/script.rb
index fa2c5a5..cf1ebf8 100644
--- a/lib/rscons/script.rb
+++ b/lib/rscons/script.rb
@@ -95,6 +95,7 @@ module Rscons
 
       # Create an environment.
       def env(*args, &block)
+        Rscons.application.check_configure
         Environment.new(*args, &block)
       end
 
@@ -250,6 +251,11 @@ module Rscons
         Rscons.application.variant(*args)
       end
 
+      # Check if a variant is enabled.
+      def variant_enabled?(*args)
+        Rscons.application.variant_enabled?(*args)
+      end
+
       # Create a variant group.
       def variant_group(*args, &block)
         Rscons.application.variant_group(*args, &block)
@@ -257,6 +263,7 @@ module Rscons
 
       # Iterate through variants.
       def with_variants(&block)
+        Rscons.application.enable_variants
         Rscons.application.with_variants(&block)
       end
 
diff --git a/spec/build_tests_spec.rb b/spec/build_tests_spec.rb
index 2932c8a..18e24c3 100644
--- a/spec/build_tests_spec.rb
+++ b/spec/build_tests_spec.rb
@@ -3100,10 +3100,64 @@ EOF
       expect(result.status).to_not eq 0
     end
 
-    it "raises an error when with_variants is called with an empty variant group" do
+    it "allows specifying the exact enabled variants on the command line 1" do
       test_dir "variants"
-      result = run_rscons(args: %w[-f error_with_variants_with_empty_variant_group.rb])
-      expect(result.stderr).to match %r{Error: empty variant group found}
+      result = run_rscons(args: %w[-v -f multiple_groups.rb -e kde,debug])
+      expect(result.stderr).to eq ""
+      expect(result.status).to eq 0
+      expect(File.exist?("build/prog-kde-debug/prog.exe")).to be_truthy
+      expect(File.exist?("build/prog-kde-release/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-gnome-debug/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-gnome-release/prog.exe")).to be_falsey
+    end
+
+    it "allows specifying the exact enabled variants on the command line 2" do
+      test_dir "variants"
+      result = run_rscons(args: %w[-v -f multiple_groups.rb -e kde,gnome,release])
+      expect(result.stderr).to eq ""
+      expect(result.status).to eq 0
+      expect(File.exist?("build/prog-kde-debug/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-kde-release/prog.exe")).to be_truthy
+      expect(File.exist?("build/prog-gnome-debug/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-gnome-release/prog.exe")).to be_truthy
+    end
+
+    it "allows disabling a single variant on the command line" do
+      test_dir "variants"
+      result = run_rscons(args: %w[-v -f multiple_groups.rb --variants=-kde])
+      expect(result.stderr).to eq ""
+      expect(result.status).to eq 0
+      expect(File.exist?("build/prog-kde-debug/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-kde-release/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-gnome-debug/prog.exe")).to be_truthy
+      expect(File.exist?("build/prog-gnome-release/prog.exe")).to be_truthy
+    end
+
+    it "allows turning off variants by default" do
+      test_dir "variants"
+      result = run_rscons(args: %w[-v -f default.rb])
+      expect(File.exist?("build/prog-debug/prog.exe")).to be_falsey
+      expect(File.exist?("build/prog-release/prog.exe")).to be_truthy
+    end
+
+    it "allows turning on an off-by-default-variant from the command line" do
+      test_dir "variants"
+      result = run_rscons(args: %w[-v -f default.rb -e +debug])
+      expect(File.exist?("build/prog-debug/prog.exe")).to be_truthy
+      expect(File.exist?("build/prog-release/prog.exe")).to be_truthy
+    end
+
+    it "allows only turning on an off-by-default-variant from the command line" do
+      test_dir "variants"
+      result = run_rscons(args: %w[-v -f default.rb -e debug])
+      expect(File.exist?("build/prog-debug/prog.exe")).to be_truthy
+      expect(File.exist?("build/prog-release/prog.exe")).to be_falsey
+    end
+
+    it "exits with an error if no variant in a variant group is activated" do
+      test_dir "variants"
+      result = run_rscons(args: %w[-v -f multiple_groups.rb --variants=kde])
+      expect(result.stderr).to match %r{No variants enabled for variant group}
       expect(result.status).to_not eq 0
     end
   end