In this chapter, we address the active processes relating to cell mechanics, where the biology and mechanics become inseparable. In contrast to the previous two chapters, this one will be more qualitative, and the models, to the extent they exist, more ad hoc. This is because not only are the processes much more complex, often involving a cascade of reactions or numerous individual cell functions, but they are also less well understood. We begin this chapter with a discussion of the various types of active cell processes involving motility in some form. These range from the motion of cilia and flagella, to phagocytosis, to cell migration along a substrate. Other phenomena on a smaller scale provide the energy for these motions, as discussed more fully in Section 1. Models for cell motility will be described next, and then the methods that have been developed to quantify it. We also include in this chapter a description of muscle and active cell contraction, beginning with a macroscopic perspective, but extending down to the level of individual cross-bridge dynamics and the models that are used to describe it. This chapter ends with a discussion of mechanotransduction. Contrary to most of the literature on this topic, however, the focus here is on the mechanisms by which force is transduced into a chemical signal, rather than on the subsequent signaling cascade that leads to the ultimate response of the cell. Because these are poorly understood, and the hypotheses still require validation, this section should be viewed as a basis for further study, and not a definitive description of known phenomena. This remains one of the most challenging, and fascinating, areas of biomechanics research.