Recent experimental and modeling work concerning Homogeneous Charge Compression Ignition (HCCI) combustion in small scales is presented. A zero-dimensional numerical model incorporating detailed chemical kinetics, heat transfer, blow-by, and a force balance is developed to interpret the experimental results and to explore HCCI combustion with a free-piston. The model consists of a new "Reactor Problem" for the sensitivity and kinetics component (Senkin) of the Chemkin software package. Following validation and interpretation of the experimental results, the model is used to conduct parametric studies. These studies and a non-dimensionalization of the governing equations yield parameters that characterize free-piston dynamics. These parameters are subsequently used to relate initial conditions to the percent mass lost. (C) 2003 The Combustion Institute. All rights reserved.