This publication has been revised to reflect updates to version 4.0 of the BehavePlus software. It was originally published as the BehavePlus fire modeling system, version 4.0: Variables in July, 2008. The BehavePlus fire modeling system is a computer program based on mathematical models that describe wildland fire behavior and effects and the fire environment. It is a flexible system that produces tables, graphs, and simple diagrams. It can be used for a host of fire management applications including projecting the behavior of an ongoing fire, planning prescribed fire, fuel hazard assessment, and training. The BehavePlus program automatically creates a worksheet that requests the required input variables based on the modules, output variables, and options selected by the user. This is a reference paper that describes the 189 variables in BehavePlus, with information on input and output relationships. A User's Guide describes operation of the program and can be accessed at https://www.fs.usda.gov/treesearch/pubs/5488.