In computer-aided design (CAD), engineers use computers to design, analyze, model, simulate, and evaluate a product, process, or system. In this chapter, the application of CAD in the creations of virtual models of products or systems is focused on. CAD was originally developed to represent geometric information of objects by using computer-drawing software. With the continuous evolution in several decades, the capabilities of CAD tools have been expanded significantly beyond computer graphics. In this chapter, various CAD techniques are introduced to model products, processes, and systems at different levels and aspects, parametric modeling, and knowledge-based engineering (KBE) are specially introduced, and basic solid modeling techniques are discussed to model geometrics, features, design intents, and assembling relations; finally, kinematic and motion analyses are explored to machine designs.