The Reactive Extensions (Rx) is a tool for composing asynchronous and event-based programs using observable sequences. It's very powerful technique to avoid callback hell and it can be used both on the server side as well as for designing the user interfaces. It leverages the concepts from functional programming such as composable monadic functors, immutable state, etc. There are libraries for Rx to almost all modern languages (Java, .Net, JavaScript, C++, Scala, Android sdk, etc.) and the presentation will show the examples in multiple languages.