Shape memory alloys (SMA) produce the largest anomalous reversible strains of all crystalline materials: epsilon(SM) < 10% in comparison to ε(FM) < 1.0% and epsilon(FE) < 0.1% (FM magnetostrictive, FE electrostrictive). They are considered as active materials for applications in which large scale motions are required or because of their high damping capacity for mechanical vibrations. Their use is however limited if a large number of cycles is applied. A survey is given on fatigue phenomena in SMA:1. classical mechanical fatigue - accumulation of defects, formation and growth of cracks,2. shape memory fatigue - change of transformation temperatures, reduction or loss of memory, loss of pseudo elasticity or damping capacity.Finally, it is shown how a modification of microstructure by thermo-mechanical treatments (TMT) affects fatigue resistance. In this context primary defects ( as introduced by TMT) and secondary defects ( introduced by mechanical or thermal cycling) have to be distinguished. In case of training for the two-way effect secondary defects are introduced voluntarily. (C) 2004 Kluwer Academic Publishers.