Abstract The decision to use a new material has technical, economical, and environmental consequences. The goal of this chapter is to help clarify quickly the use, possibilities, and problems of thermoplastics and thermoplastic composites. We compare consumptions, engineering and specific properties, costs of plastics, and composites with metals and some other conventional materials (wood, glass, paper, textile). After some basic reminders concerning thermoplastics, thermosets, composites, thermoplastic elastomer, 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 much iteration concerning the functional properties, the environmental constraints, the possibilities to produce the part in the required quantities, and the price.