A Mathematical model describing the heat transfer and fluid hydrodynamics during the engagement cycles of “wet” clutches have been developed. The model equations of heat balance for the separator, friction materials, core plate, and automatic transmission oil (ATF) or air in the clearance between separators and friction materials were formulated. The momentum balance for ATF flow in the friction lining (porous medium) during the engagement was considered. Proper boundary conditions at the interface and boundary conditions at the inner and outer radius of the clutches were accounted for. The model includes the simulation of engagement cycles for both the traditional (two-sided) wet clutches and the single-sided wet clutches. Many important phenomena which were not investigated before, such as non-uniform contact between separators and friction materials, ATF flow into the friction materials due to liquid permeability, compressibility of friction materials, effects of separator splines, and effects of rotational speeds, etc, were included in the present model. The effect of various important factors affecting the interface temperatures have been examined. Two of the important input parameters to the model, namely the compression modulus and liquid permeability of various types of friction materials, have been determined experimentally. The results of numerical simulation on interface temperature are in good agreement with the experimental measurements on interface temperature.