The design of systems requires the integration of data from different fields (mechanical computer-aided design, electrical computer-aided design, automation, embedded software,.). The multi-technological character of Mechatronics makes the design of systems more complex. In this context a general knowledge of Mechatronics and Mechatronic products is necessary. The design problem of a mechatronic product requires the analysis of its structural, technological, and functional complexity. This paper presents an approach to the design of Mechatronic systems. This requires the characterization of the different types of the complexity of a product before presenting the design methodology, the modeling, and the simulation tools used in the different design phases.