Introduction Structural materials are generally divided into four basic categories: metals@ ceramics@ polymers@ and composites. This scheme is based primarily upon the microstructures and chemical makeups of the materials [1-1]. Metals are combinations of metallic elements and contain nonlocalized electron . Metals@ including steels and nonferrous metals such as aluminum@ are generally strong and yet deformable. Ceramics are compounds of metallic and nonmetallic elements. They are highly crystalline oxides@ nitrides@ or carbides@ and therefore are very stiff and brittle. Polymers are composed of very large molecular chains of carbon atoms that are side-bonded with various atoms or radicals. The long molecular chains are mostly bonded with weak van der Waals forces@ which make polymers neither as strong nor as stiff as metals or ceramics. However@ the long molecular structures provide polymers with high exibility. Composites are arti cially produced materials that consist of two or more separate materials combined in a macroscopic structural unit. Unlike these traditional materials (metals@ ceramics@ and polymers) whose microstructures are relatively xed@ composites are highly tunable from microstructure and mechanical properties points of view. As a result@ composites can have a desirable combination of the best properties of the constituent phases@ meaning they can be strong and ductile@ stiff@ and lightweight. Composites have been successfully used in aerospace and space industries and are gaining momentum in the automotive industry. With a yield strength of more than ten times that of steel or aluminum@ and a density of only about one- fth that of steel and one-half that of aluminum@ composites have become the top choice for producing lightweight vehicles [1-2]. According to the U.S. Department of Energy@ the transportation sector accounts for 28% of total U.S. energy use@ two-thirds of the nation's petroleum consumption@ and a third of the nation's carbon emissions [1-3]. Further@ the transportation industry accounts for nearly 32% of U.S. greenhouse gas emissions [1-4]. It is known that the fuel consumption is directly related to the vehicle weight [1-5]. Reductions in vehicle weight can be achieved by a combination of (1) vehicle downsizing@ (2) vehicle redesign and contents reduction@ and (3) material substitution [1-6]. Among these three options@ substituting heavy metallic materials with strong and light composites seems to be the most viable choice. The bene ts of composites go far beyond weight savings. Polymer matrix composites have great potential for part integration@ which will result in lower manufacturing costs and faster time to market. The composite parts can have much smaller tooling costs as compared to metal ones. Composites also have much better corrosion resistance than metals and are more resistant to damage@ such as dents and dings@ than aluminums. Polymer composites possess superior viscoelastic damping and thus provide improved noise@ vibration@ and harshness (NVH) performance to the vehicles. Composites also have a high level of styling exibility in terms of deep drawn panel@ which goes beyond what can be achieved with metal stampings. Although the bene ts of composites are well recognized by the industry@ the use of composites has been hampered by numerous factors. These mainly include the high costs associated with raw materials and manufacturing and also the lack of design knowledge with composites [1-7] [1-8]. The automotive industry has traditionally worked with isotropic materials such as steels and aluminums. For anisotropic composites@ there is a dearth of design data (material property database)@ design tools (models)@ testing methods@ and nally the design examples. This book focuses on the design aspects of the composite materials. It begins with a brief introduction to composite materials and design process and then presents some of the most recent@ innovated design examples of composite structures by engineers from the automobile OEMs and top-tier suppliers.