Skeletal muscle is the actuator of motor programs generated by the central nervous system (CNS). The CNS fine-tuning of muscles’ activity makes very complex motor patterns appear extremely simple and smooth. This chapter will provide a synthetic summary of the most up-to-date knowledge in the field of muscle contractile properties and the relationships between muscle structure and function. To date, despite the large amount of information concerning muscle structure and function, the equations describing muscle mechanical behavior are yet to be fully defined, which can be largely attributed to the fact that the real muscle exerts its action along with other mechanical structures. Muscle forces must be transmitted to bones through the muscle-tendon junction and the tendon. The mechanical properties of such elements may influence muscle force exertion and its mechanics, thus the necessity for developing models of muscle mechanics that are continuously updated with new results provided by the experiments. However, to respect the boundaries of this section, we will not refer either to CNS control of muscle activity or to the mechanical properties of noncontractile elements. Also, the energetic aspects of muscle contraction will not be discussed in this chapter.