FEFLOW is a software package for modeling fluid flow and transport of dissolved constituents and/or heat transport processes in the subsurface. FEFLOW contains pre- and post processing functionality and an efficient simulation engine. A user-friendly graphical interface provides easy access to the extensive modeling options. FEFLOW - in contrast to some of the competing products - is not a graphical front end for a separately developed simulation kernel. It is a completely integrated system from simulation engine to graphical user interface. It includes a public programming interface for user code. FEFLOW is developed by WASY GmbH, a German company. WASY's areas of expertise encompass groundwater hydrology, surface water hydrology and geographic information systems. In these fields, WASY provides software, training and consulting services. FEFLOW is used by leading research institutes, universities, consulting firms and government organizations all over the world. Its scope of application ranges from simple local-scale to complex large-scale simulations.