RSiena is a free, open source social network analysis package for R. It replaces Siena (Simulation Investigation for Empirical Network Analysis), which was a stand-alone program for Windows. Like its predecessor, RSiena is optimized for social network data, but is also applicable to other forms of network data, and relies on Markov chain models to generate network dynamics. This approach is best applied to data where relations between actors and their behaviour can be treated as a state rather than as an event. Users not familiar with R can run RSiena via the provided graphical user interface.