A new ignition and combustion model has been developed and tested for use in premixed spark-ignition engines. The ignition model is referred to as the Discrete Particle Ignition Kernel (DPIK) model, and it uses Lagrangian markers to track the flame-front growth. The model includes the effects of electrode heat transfer on the early flame kernel growth process, and it is used in conjunction with a characteristic-time-scale combustion model once the ignition kernel has grown to a size where the effects of turbulence on the flame must be considered. A new term which accounts for the effect of air-fuel ratio, was added to the combustion model for modeling combustion in very lean and very rich mixtures. The flame kernel size predicted by the DPIK model was compared with measurements of Maly and Vogel. Furthermore, predictions of the electrode heat transfer were compared with data of Kravchik and Heywood. In both comparisons the model predictions were in good agreement with the experiments. A parametric study showed that heat transfer to the electrodes is important during the early kernel growth process. The ignition and combustion models were applied to a homogeneous charge Caterpillar SI engine with four different spark-timings, and good agreement with experimental cylinder pressure data was obtained. In addition, a premixed-charge Briggs-Stratton engine was modeled over a range of air-fuel ratios from 11-19. Results from both engines demonstrated that with a more sophisticated ignition model, multidimensional model predictions of spark-ignition engine combustion can be improved significantly.