Siena (Simulation Investigation for Empirical Network Analysis) is a free program for statistical analysis developed particularly to work with social network data, but also applicable to other forms of network data. It models network dynamics via Markov chain models; 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. Siena's statistical analysis processes can be time-consuming, as the system works via computer simulation of the network. It was in continuous development from 2001 through 2010, releasing versions up to 3.2. Development on Siena as a stand-alone program has concuded, and the project has shifted focus to RSiena, a version of Siena deployed in R.