This chapter is dedicated to the presentation of composites and other hybrid structures. It introduces several promising reinforcement constructions, often produced by textile manufacturing methods such as weaving, knitting, braiding, or hybrid technologies. The industrial use and development of two largely used synthetic fibers that are glass and carbon fibers is introduced. The environmental regulation REACH (Registration, Evaluation, Authorization and restriction of Chemicals) application reduced strongly the use of aramid fibers even if they are still principally used for some military and security applications such as armors and helmets. The aramid fibers are only shortly mentioned and presented. The presentation of natural fibers, which are attracting more and more attention from automotive sports and leisure and building and construction sectors, is given in this chapter. Following the classification of composites in function of their matrix to three distinct classes: (1) Metal Matrix Composites (MMC), (2) Ceramic Matrix Composites (CMC), and (3) Plastic Matrix Composites (PMC) and a relatively large diffusion of PMCs comparing to MMC and CMC that can be explained with an excellent ratio cost/performances; the chapter focuses on PMCs and their monitoring in real time in situ, using smart textile methodology applied to composites' reinforcements. This part ends by an overview of failure mechanisms in composites and an introduction to hybrid structures, their production methodology, bonding issues, and properties.