ExtendSim is a simulation program for modeling discrete event, continuous, agent-based, discrete rate, and mixed-mode processes. There are three main ExtendSim simulation model building packages: CP for modeling continuous processes; DE which adds discrete event technology; and Pro which adds discrete rate and reliability block diagramming modules.

Models are created by placing blocks from a library into a model worksheet. Blocks are connected together to create the logical flow of the model. Data for the model resides in the parameters of the blocks and in a proprietary database. New blocks can be created by combining existing blocks into a single hierarchical block or by programming a block in ExtendSim's C-based language, ModL. The major libraries in ExtendSim are:

Sample applications include resource optimization for food logistics, six sigma process improvement for a hospital emergency department, communication systems, and manufacturing facility design