diff --git a/test/test_instrumentation.rb b/test/test_instrumentation.rb
new file mode 100644
index 0000000..27308d8
--- /dev/null
+++ b/test/test_instrumentation.rb
@@ -0,0 +1,187 @@
+require 'minitest/autorun'
+require 'prism'
+require 'stringio'
+
+class InstrumentationTest < Minitest::Test
+  def setup
+    @bus = Prism::Instrumentation::Bus.new
+  end
+
+  def test_instrument_returns_block_value
+    result = @bus.instrument('demo.event') { 42 }
+    assert_equal 42, result
+  end
+
+  def test_subscriber_receives_event_with_name_and_payload
+    events = []
+    @bus.subscribe('demo.event') { |event| events << event }
+    @bus.instrument('demo.event', foo: 'bar') { :ok }
+
+    assert_equal 1, events.size
+    assert_equal 'demo.event', events.first.name
+    assert_equal 'bar', events.first.payload[:foo]
+  end
+
+  def test_duration_is_a_non_negative_float
+    events = []
+    @bus.subscribe('demo.event') { |event| events << event }
+    @bus.instrument('demo.event') { sleep 0.01 }
+
+    assert_kind_of Float, events.first.duration
+    assert_operator events.first.duration, :>=, 0.01
+    assert_operator events.first.duration_ms, :>=, 10.0
+  end
+
+  def test_string_pattern_matches_exact_name_only
+    events = []
+    @bus.subscribe('demo.event') { |event| events << event }
+    @bus.instrument('other.event') { :ok }
+
+    assert_empty events
+  end
+
+  def test_regexp_pattern_matches_by_suffix
+    events = []
+    @bus.subscribe(/\.prism\z/) { |event| events << event }
+    @bus.instrument('sql.prism') { :ok }
+    @bus.instrument('unrelated.event') { :ok }
+
+    assert_equal ['sql.prism'], events.map(&:name)
+  end
+
+  def test_nil_pattern_matches_everything
+    events = []
+    @bus.subscribe { |event| events << event }
+    @bus.instrument('a') { :ok }
+    @bus.instrument('b') { :ok }
+
+    assert_equal %w[a b], events.map(&:name)
+  end
+
+  def test_multiple_subscribers_all_notified
+    calls = []
+    @bus.subscribe('demo.event') { calls << :first }
+    @bus.subscribe('demo.event') { calls << :second }
+    @bus.instrument('demo.event') { :ok }
+
+    assert_equal %i[first second], calls
+  end
+
+  def test_unsubscribe_stops_future_notifications
+    events = []
+    subscriber = @bus.subscribe('demo.event') { |event| events << event }
+    @bus.instrument('demo.event') { :ok }
+    @bus.unsubscribe(subscriber)
+    @bus.instrument('demo.event') { :ok }
+
+    assert_equal 1, events.size
+  end
+
+  def test_block_can_mutate_payload_before_publish
+    events = []
+    @bus.subscribe('demo.event') { |event| events << event }
+    @bus.instrument('demo.event', {}) { |payload| payload[:computed] = 'late' }
+
+    assert_equal 'late', events.first.payload[:computed]
+  end
+
+  def test_exception_propagates_and_is_recorded_on_payload
+    events = []
+    @bus.subscribe('demo.event') { |event| events << event }
+
+    assert_raises(RuntimeError) do
+      @bus.instrument('demo.event') { raise 'boom' }
+    end
+
+    assert_equal 1, events.size, 'event must still publish when the block raises'
+    assert_equal ['RuntimeError', 'boom'], events.first.payload[:exception]
+    assert_kind_of RuntimeError, events.first.payload[:exception_object]
+  end
+
+  def test_instrument_without_block_returns_nil_and_still_publishes
+    events = []
+    @bus.subscribe('demo.event') { |event| events << event }
+    result = @bus.instrument('demo.event')
+
+    assert_nil result
+    assert_equal 1, events.size
+  end
+
+  def test_concurrent_instrument_and_subscribe_do_not_corrupt_state
+    counter = 0
+    mutex = Mutex.new
+    @bus.subscribe('demo.event') { mutex.synchronize { counter += 1 } }
+
+    threads = Array.new(20) do
+      Thread.new { 50.times { @bus.instrument('demo.event') { :ok } } }
+    end
+    threads.each(&:join)
+
+    assert_equal 1000, counter
+  end
+
+  def test_module_level_bus_is_a_singleton
+    assert_same Prism::Instrumentation.bus, Prism::Instrumentation.bus
+  end
+
+  def test_module_level_instrument_and_subscribe_delegate_to_bus
+    events = []
+    subscriber = Prism::Instrumentation.subscribe('module.demo') { |event| events << event }
+    Prism::Instrumentation.instrument('module.demo') { :ok }
+    Prism::Instrumentation.unsubscribe(subscriber)
+
+    assert_equal 1, events.size
+  end
+end
+
+class InstrumentationLogSubscriberTest < Minitest::Test
+  def setup
+    @bus = Prism::Instrumentation::Bus.new
+    @io = StringIO.new
+  end
+
+  def test_attach_writes_one_formatted_line_per_event
+    subscribers = attach_to(@bus, @io, patterns: ['sql.prism'])
+    @bus.instrument('sql.prism', operation: 'SELECT', table: 'posts') { :ok }
+    subscribers.each { |s| @bus.unsubscribe(s) }
+
+    line = @io.string.lines.first
+    assert_match(/sql\.prism \(\d+\.\d ms\)/, line.gsub('ms)', ' ms)'))
+    assert_match(/operation="SELECT"/, line)
+    assert_match(/table="posts"/, line)
+  end
+
+  def test_attach_ignores_events_not_matching_pattern
+    attach_to(@bus, @io, patterns: ['sql.prism'])
+    @bus.instrument('render_template.prism', identifier: 'posts/index') { :ok }
+
+    assert_empty @io.string
+  end
+
+  def test_attach_hides_exception_object_from_output
+    attach_to(@bus, @io, patterns: ['demo.event'])
+    assert_raises(RuntimeError) { @bus.instrument('demo.event') { raise 'boom' } }
+
+    refute_match(/exception_object/, @io.string)
+    assert_match(/exception=\["RuntimeError", "boom"\]/, @io.string)
+  end
+
+  def test_attach_wires_to_the_process_wide_bus
+    subscribers = Prism::Instrumentation::LogSubscriber.attach(@io, patterns: ['demo.singleton'])
+    Prism::Instrumentation.instrument('demo.singleton', a: 1) { :ok }
+    subscribers.each { |s| Prism::Instrumentation.unsubscribe(s) }
+    Prism::Instrumentation.instrument('demo.singleton', a: 2) { :ok }
+
+    assert_equal 1, @io.string.lines.size
+    assert_match(/a=1/, @io.string)
+  end
+
+  private
+
+  # Most assertions here only need LogSubscriber's formatting, so they run
+  # against a throwaway Bus instead of the process-wide singleton to stay
+  # isolated from other tests; #attach itself is exercised separately above.
+  def attach_to(bus, io, patterns:)
+    patterns.map { |pattern| bus.subscribe(pattern) { |event| io.puts(Prism::Instrumentation::LogSubscriber.format_event(event)) } }
+  end
+end