Originally observed in nuclear physics, these also play an important role for ultra-cold atoms as they are prepared in magneto-optical traps and Bose-Einstein condensation. When two slow atoms collide, they usually do not stay together for very long. However, when a Feshbach resonance is observed, they stick together for a longer time. This can lead to a dramatic increase in the formation of molecules by photo-association in the trap or alter the properties of a Bose-Einstein condensate dramatically. Close to a Feshbach resonance, the atom-atom interaction is extremely sensitive to the exact shape of the potential energy curves such that small changes in, for instance, the magnetic field might switch from attractive to repulsive behavior. Feshbach resonances have typical signatures: the continuum wave-function shows a phase change of π over an energy range of the Feshbach resonance.