diff --git a/CHANGELOG.md b/CHANGELOG.md
index 01ccef2..0843b33 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,11 @@
+## Unreleased
+
+### New Features
+
+- Warn about unused rules (rules not reachable from any start rule). The
+  warnings are written to the log file and, with `-w`, are printed to stderr
+  and treated as fatal errors.
+
 ## v5.2.0
 
 ### New Features
diff --git a/README.md b/README.md
index f64f29b..de6c21d 100644
--- a/README.md
+++ b/README.md
@@ -39,8 +39,8 @@ Propane is typically invoked from the command-line as `./propane`.
                   debugging a grammar.
       --version   Show program version and exit.
       -w          Treat warnings as errors. This option will treat shift/reduce
-                  conflicts as fatal errors and will print them to stderr in
-                  addition to the log file.
+                  conflicts and unused rules as fatal errors and will print
+                  them to stderr in addition to the log file.
 
 The user must specify the path to a Propane input grammar file and a path to an
 output file.
diff --git a/doc/user_guide.md b/doc/user_guide.md
index 8c49637..79d35fd 100644
--- a/doc/user_guide.md
+++ b/doc/user_guide.md
@@ -43,8 +43,8 @@ Propane is typically invoked from the command-line as `./propane`.
                   debugging a grammar.
       --version   Show program version and exit.
       -w          Treat warnings as errors. This option will treat shift/reduce
-                  conflicts as fatal errors and will print them to stderr in
-                  addition to the log file.
+                  conflicts and unused rules as fatal errors and will print
+                  them to stderr in addition to the log file.
 
 The user must specify the path to a Propane input grammar file and a path to an
 output file.
diff --git a/lib/propane/cli.rb b/lib/propane/cli.rb
index 5a5b5af..bc0a703 100644
--- a/lib/propane/cli.rb
+++ b/lib/propane/cli.rb
@@ -10,8 +10,8 @@ Options:
               debugging a grammar.
   --version   Show program version and exit.
   -w          Treat warnings as errors. This option will treat shift/reduce
-              conflicts as fatal errors and will print them to stderr in
-              addition to the log file.
+              conflicts and unused rules as fatal errors and will print
+              them to stderr in addition to the log file.
 EOF
 
     class << self
diff --git a/lib/propane/parser.rb b/lib/propane/parser.rb
index 56432b9..19d902c 100644
--- a/lib/propane/parser.rb
+++ b/lib/propane/parser.rb
@@ -50,6 +50,7 @@ class Propane
 
       build_reduce_actions!
       build_tables!
+      check_unused_rules!
       write_log!
       errormessage = ""
       if @errors.size > 0
@@ -68,6 +69,34 @@ class Propane
 
     private
 
+    # Record a warning for each user-defined rule that cannot be reached from
+    # any start rule.
+    #
+    # @return [void]
+    def check_unused_rules!
+      reachable = Set.new
+      pending = @grammar.rules[0...@grammar.start_rules.length].map(&:rule_set)
+      while rule_set = pending.pop
+        next if reachable.include?(rule_set)
+        reachable << rule_set
+        rule_set.rules.each do |rule|
+          rule.components.each do |component|
+            if component.is_a?(RuleSet) && !reachable.include?(component)
+              pending << component
+            end
+          end
+        end
+      end
+      @rule_sets.each do |name, rule_set|
+        # Skip rule sets generated internally (real start rules and optional
+        # component rules).
+        next if name.start_with?("$") || rule_set.optional?
+        unless reachable.include?(rule_set)
+          @warnings << "Unused rule #{name} (defined on line #{rule_set.rules.first.line_number})"
+        end
+      end
+    end
+
     def build_tables!
       @state_table = []
       @shift_table = []
diff --git a/spec/propane_spec.rb b/spec/propane_spec.rb
index 471a22e..5d5dea9 100644
--- a/spec/propane_spec.rb
+++ b/spec/propane_spec.rb
@@ -267,6 +267,50 @@ EOF
     expect(File.binread("spec/run/testparser.log")).to match %r{Shift/Reduce conflict \(state \d+\) between token b and rule As2\? \(defined on line 4\)}
   end
 
+  it "warns on unused rules" do
+    write_grammar <<EOF
+token a;
+token b;
+Start -> a;
+Unused -> b Other?;
+Other -> a;
+EOF
+    results = run_propane(capture: true)
+    expect(results.stderr).to eq ""
+    expect(results.status).to eq 0
+    log = File.binread("spec/run/testparser.log")
+    expect(log).to match %r{Unused rule Unused \(defined on line 4\)}
+    expect(log).to match %r{Unused rule Other \(defined on line 5\)}
+    expect(log).to_not match %r{Unused rule (Start|Other\?)}
+  end
+
+  it "errors on unused rules with -w" do
+    write_grammar <<EOF
+token a;
+token b;
+Start -> a Used;
+Used -> b;
+Unused -> b;
+EOF
+    results = run_propane(extra_args: %w[-w], capture: true)
+    expect(results.stderr).to match %r{Unused rule Unused \(defined on line 5\)}
+    expect(results.stderr).to_not match %r{Unused rule (Start|Used)\b}
+    expect(results.status).to_not eq 0
+  end
+
+  it "does not warn about rules reachable from any of multiple start rules" do
+    write_grammar <<EOF
+token a;
+token b;
+start Start Other;
+Start -> a;
+Other -> b;
+EOF
+    results = run_propane(extra_args: %w[-w], capture: true)
+    expect(results.stderr).to eq ""
+    expect(results.status).to eq 0
+  end
+
   it "errors on duplicate field aliases in a rule" do
     write_grammar <<EOF
 token a;