AbstractThe ability of polymers to respond to external stimuli such as heat or light is of high scientific and technological significance. Their stimuli-sensitive behavior enables such materials to change certain macroscopic properties such as shape, color or refractive index controlled by an external signal. The implementation of the capability to actively move into polymers has attracted researcher’s interest, especially in the last years, and was achieved in polymers as well as gels. Sensitivity to heat, light, magnetic fields, ion strength or pH value was realized in gels . In non-swollen polymers active movement is stimulated by exposition to heat or
light and could also be designed as a complex movement with more than two shapes.
Besides their scientific significance such materials have an high innovation potential and can be found e.g. in smart fabrics [2-4], heat-shrinkable tubes for electronics or films for packaging , self-deployable sun sails in space crafts , self-disassembling mobile phones
, intelligent medical devices  or implants for minimally invasive surgery [9-11]. These enumerations are only exemplarily and cover only a small amount of potential applications. Actively moving polymers may even reshape product design . In this chapter different
classes of actively moving materials are introduced with an emphasis on shape-memory polymers. The fundamental principles of the different functions are explained and examples for specific materials are given.