the two techniques, called play-in, is a convenient way to 'play in' scenario­ based behavior directly from the system's graphical user interface (QUI). The second technique, play-out, makes it possible to execute, or 'play out', the behavior on the QUI as if it were programmed in a conventional intra­ object state- based fashion. All this is implemented in full in our tool, the Play-Engine. The book can be viewed as offering improvements in some ofthe phases of known system development life cycles, e.g., requirements capture and anal­ ysis, prototyping, and testing. However, there is a more radical way to view the book, namely, as proposing an alternative way to program reactivity, which, being based on inter-object scenarios, is a lot closer to how people think about systems and their behavior.