BfK is computer based program for simulation of source strength, concentration and casualty assessments when a chemical is released. OS platform can be MS Windows 95/98/NT4/2000. To calculate the concentration of a chemical following a certain release, a number of physical models are meeded. These models have already been chosen or developed by FOA but some adjustments have been made to make them better suited for being implemented in a computer program. To be able to document the source code in a proper way this report is written during the process of the implementation. This report describes how different components in the BfK program work. It mainly describes the physical models and discusses them, for example limitations and ideas of future improvements.