Publisher Summary This chapter provides an understanding of the real applications for thermoplastics and thermoplastic composites and the relative importance of the various material families and processes involved. It discusses the industrial and economic reality of polymers as compared to traditional materials. The specific mechanical properties take account of the density and consider the performance to density ratio. Polymers are sensitive to a greater or lesser degree to photo-degradation, which can limit their exterior uses. Polymers, like other materials, are sensitive to fatigue. The study presents some examples of fatigue test results according to the logarithm of the number of cycles leading to failure. Designers are directly or indirectly subjected to economical, technical and environmental constraints. Plastics and composites are well positioned to provide solutions. Furthermore, the technical and economic possibilities of processing are outlined. Polymer recycling presents technical and economic difficulties and is less advanced, industrially, than that of metals. The final choice of the design team results from many iterations concerning the functional properties, the environmental constraints, the possibilities to produce the part in the required quantities, and the price.