The main motivations of using smart materials as fundamental in micro/ nanopositioning systems are presented in this chapter. It is shown that the design of classical (or macro) actuated systems cannot directly be used to design small ones, particularly those used for micro/nanopositioning. While in macro, many components are assembled to form the actuated systems, in micro one attempts to reduce the number of elements in order to ensure some resolution and accuracy of positioning and in order to make easy their fabrication. Smart and active materials are therefore seen as the principal and essential component in microsystems and systems working at the micro/nano-scale. Their advantages are detailed in the chapter and some of the behaviors (hysteresis and creep) that are often encountered are explained. A particular attention is given to piezoelectric materials since nine chapters of the book treat them.