Abstract The decision to use a new material has technical, economic, and environmental consequences. The goal of this chapter is to help clarify quickly the use, possibilities, and problems of thermosets and composites. Consumptions, engineering and specific properties, costs of plastics and composites, are compared with metals and some other conventional materials. After some basic reminders concerning thermosets, thermoplastics, composites, and related processing methods, the designer’s main problems are examined, taking into account the economic, technical, marketing, sustainability, and environmental requirements. Recycling, plastics of renewable sources and some weaknesses of polymer materials end this chapter. The final choice of the design team will result from many iterations concerning the functional properties, the environmental constraints, the possibilities to produce the part in the required quantities, and the price. Market study specialists, associations and institutes are quoted to ease in-depth studies by the reader.