diff --git a/lib/prism/mailer/preview_app.rb b/lib/prism/mailer/preview_app.rb
new file mode 100644
index 0000000..97be41e
--- /dev/null
+++ b/lib/prism/mailer/preview_app.rb
@@ -0,0 +1,99 @@
+module Prism
+  module Mailer
+    # A tiny Rack app that browses Mailer::Preview subclasses in a web
+    # browser, mirroring Rails' /rails/mailers engine: an index of every
+    # registered preview, a list of its actions, and a rendered view of one
+    # email (HTML part if present, raw text part otherwise) - all without
+    # ever touching Mailer.deliver!. Mount it directly:
+    #   BlogApp.mount('/rails/mailers', Prism::Mailer::PreviewApp.new)
+    class PreviewApp
+      def call(env)
+        request = Request.new(env)
+        return not_found unless request.request_method == 'GET'
+
+        # Links are built from SCRIPT_NAME (where this app was mounted)
+        # rather than relative to PATH_INFO, so they work the same whether
+        # this is mounted at "/rails/mailers" or served standalone at "/" -
+        # relative hrefs would break the moment the current path lacks a
+        # trailing slash (the classic Rack::URLMap gotcha).
+        @base = env['SCRIPT_NAME'].to_s
+        segments = request.path.split('/').reject(&:empty?)
+        case segments.length
+        when 0 then index
+        when 1 then show_preview(segments[0])
+        when 2 then show_email(segments[0], segments[1])
+        else not_found
+        end
+      rescue ArgumentError
+        not_found
+      end
+
+      private
+
+      def index
+        previews = Preview.registry.sort_by(&:preview_name)
+        items = previews.map do |klass|
+          %(<li><a href="#{@base}/#{klass.preview_name}">#{escape(klass.preview_name)}</a></li>)
+        end
+        body = "<h1>Mailer previews</h1><ul>#{items.join}</ul>"
+        body = '<h1>Mailer previews</h1><p>No previews registered.</p>' if previews.empty?
+        html(body)
+      end
+
+      def show_preview(name)
+        klass = Preview.find(name)
+        return not_found unless klass
+
+        items = klass.emails.map do |email|
+          %(<li><a href="#{@base}/#{klass.preview_name}/#{email}">#{escape(email)}</a></li>)
+        end
+        body = +%(<p><a href="#{@base}">&laquo; all previews</a></p>)
+        body << "<h1>#{escape(klass.preview_name)}</h1><ul>#{items.join}</ul>"
+        html(body)
+      end
+
+      def show_email(name, email)
+        klass = Preview.find(name)
+        return not_found unless klass
+
+        delivery = klass.call(email)
+        html(render_message(klass, email, delivery.message))
+      end
+
+      def render_message(klass, email, message)
+        body = +%(<p><a href="#{@base}/#{klass.preview_name}">&laquo; #{escape(klass.preview_name)}</a></p>)
+        body << "<h1>#{escape(klass.preview_name)}##{escape(email)}</h1>"
+        body << '<dl>'
+        body << "<dt>Subject</dt><dd>#{escape(message.subject)}</dd>"
+        body << "<dt>From</dt><dd>#{escape(message.from)}</dd>"
+        body << "<dt>To</dt><dd>#{escape(message.to.join(', '))}</dd>"
+        body << '</dl><hr>'
+        body << render_body(message)
+        body
+      end
+
+      def render_body(message)
+        if message.html_body
+          message.html_body
+        elsif message.text_body
+          "<pre>#{escape(message.text_body)}</pre>"
+        else
+          '<p><em>empty body</em></p>'
+        end
+      end
+
+      def escape(text)
+        CGI.escapeHTML(text.to_s)
+      end
+
+      def html(body)
+        [200, { 'Content-Type' => 'text/html; charset=UTF-8' },
+         ["<!doctype html><html><head><meta charset=\"utf-8\"></head><body>#{body}</body></html>"]]
+      end
+
+      def not_found
+        [404, { 'Content-Type' => 'text/plain' }, ['Not Found']]
+      end
+    end
+  end
+end