This paper discusses the side impact computer model,SIFEM (Side Impact Finite Element Model). The SIFEM model is being developed for use as an analytical design and evaluation tool for vehicle-to-vehicle side impacts. The SIFEM model uses two (2)two-dimensional, linked, rigid body models to represent the three dimensional movement of the occupant and a two-dimensional spring -mass model to represent the impacting vehicles. The model makes use of the elasto-plastic,finite element method to calculate the force-displacement characteristics of various spring elements in the model. This paper discusses the technical features of the model and the theoretical basis from which the model was derived.