Assembly modeling as one of the most important steps in the product development activity relies more and more on the extensive use of computer-aided design (cad) systems. The modeling of geometric interfaces between the components of the assembly is of central importance in the simulation of mechanical assemblies. Over the past decades, many researchers have devoted their efforts to establish theories and systems covering assembly modeling. Although the product form or shape has been extensively investigated considering the nominal cad geometry, inevitable limitations can be reported. Computer aided tolerancing (CAT) systems provide simulation tools for modeling the effects of tolerances on the assembly but still lack of form deviation considerations. The skin model concept which stemmed from the theoretical foundations of geometrical product specification and verification (GPS) has been developed to enrich the nominal geometry considering realistic physical shapes. However, the digital representation of the skin model has been investigated only recently. This paper presents a novel approach for a skin model based simulation of contact and mobility for assemblies. Three important issues are addressed: the geometric modeling of the contact, the contact quality evaluation, and the motion analysis. The main contribution to CAT can be found in the analysis of the effects of geometric form deviations on the assembly and motion behavior of solid mechanics, which comprises models for the assembly simulation, for the contact quality evaluation, and for the motion analysis. A case study is presented to illustrate the proposed approaches.