CHEMKIN is highly structured and modular, consisting of three basic utilities packages: Gas Kinetics, Surface Kinetics, and Transport. Gas Kinetics forms the core set of utilities, upon which the other two depend. Use of these utilities requires the manipulation of a number of programs, subroutines, and data files. This section describes the structure of the Gas-phase Kinetics package and the basic requirements for using it from within your own application programs. Structure and Use of the Surface Kinetics Utilities provides similar information for the Surface Kinetics utilities, while Structure and Use of the Transport Utilities provides an overview of the Transport utilities.