The design of smart materials based on different natural and synthetic polymers represents one of the most attractive research areas over the last several years. Many efforts have been carried out to find new solutions for developing stimuli-responsive polymers sensitive to different triggers, which have a fast and reversible nonlinear response to small changes of external stimuli or environmental conditions. In combination with other organic/innorganic/macromolecular compounds, new composite materials owning multifunctional properties can be designed. Thus, the inorganic nanoparticles incorporated into a polymer matrix provide novel functionalities to hydrogels and tunable properties. This book reviews recent advances and developments of various classes of stimuli responsive polymer materials, different type of hydrogels for biomedical and pharmaceutical devices, scaffolds for tissue regeneration, and stimuli-responsive sensors or separation processes.