Crossbar.io is an open-source server platform which enables real-time communication between application components and devices in IoT, mobile and Web applications. Using Crossbar.io, application developers concentrate on the value-added functionality of their specific solution and realize the advantages of an agile application architecture consisting of loosely coupled components. Application components can be written in different programming languages (e.g. JavaScript in the browser and under NodeJS, Python, PHP, Java, C#, C++ and ObjectiveC) and interact over flexible communication patterns (Remote Procedure Calls und Publish & Subscribe). Crossbar.io is designed for high performance, scalability, stability and security. Crossbar.io is based on open Web standards (WebSocket and WAMP), an open community ecosystem and has a growing, active developer community. Typical application areas include IoT solutions in the industrial sector, health care, building- and home-automation, logistics and supply chains, smart city, finance and trading platforms, telecommunication and real-time enabled Web interfaces.