The development of a modern internal combustion engine can be characterized by three main trends: durability increase, emission reduction, and fuel economy improvement. Ring pack design addresses all of these issues. The ring behavior affects the blow-by past the ring pack, the oil film left on the cylinder liner, the friction force between the liner and the ring, and the wear of the ring and the cylinder liner. In order to predict these phenomena, the prediction of inter-ring gas flow and ring behavior, especially ring motion and ring twist about ring centroid, is needed. This paper presents the results of the modeling of 3-dimensional ring twist and its influence on ring performance and blow-by.The TWIST program includes a 3-dimensional beam model of a piston ring. Based on ring cross sectional properties, ring tension, pressure load above and below the ring, friction force and inertia force, the model predicts axial displacement, radial displacement, bend angle and twist angle of the ring over an engine cycle for any arbitrary ring cross section and groove geometry.