A physical model for flame kernel development in spark ignition engines has been developed. It is based on a coupled experimental and theoretical analysis of early stages of the flame kernel development. The experimental work involves high-speed imaging of chemi-luminescent combustion light during the very early stages of combustion just after ignition. The resulting sequences of flame kernel images were analyzed to yield the time dependencies of quantities such as: the total kernel growth, the thermal expansion part of growth, the local translational velocity of the centroid, the stretching of the flame kernel surface and its roughness. The theoretical part of the model is one-dimensional and derived from the conservation equations and well-known thermodynamic relations. It considers input of electrical energy, combustion energy release, heat transfer to the spark plug and to the unburned mixture. The model also takes into account the shear forces and the kinetic energy evolved by the flame kernel movement as well as the flame stretching effect. The governing equations.of the model have been solved numerically in conjunction with an advanced chemical kinetics package, CHEMKIN, which is designed to facilitate the simulation of the detailed chemical reactions and the evaluation of the coefficients of the transport properties. The concentration of each of the species participating in the combustion process, their chemical heat release, their temperature history and transport properties have all been evaluated.