Zynaptic Reaction is a flexible asynchronous programming framework for Java which may be used to implement complex event-driven applications. It is heavily influenced by the Twisted programming framework developed by TwistedMatrix Labs for the Python programming language. The focus of the Reaction library is on the concurrency and callback model and as such it is application neutral. It can be used to manage lots of concurrent I/O or to farm out compute intensive tasks to multicore processors. As well as being usable as a basic Java library, Reaction can also run as an independent OSGi service and integrate into any GUI framework you choose.