Coiled springs (mainsprings) are coiled steel ribbons that have been used as a power source for centuries. A major disadvantage of mainsprings is that they do not deliver a uniform torque during unwinding. Theoretical analysis of mainsprings is notoriously difficult, and they are usually designed using well established rules of thumb. In service mainsprings are subjected to low cycle fatigue loading. They can fail either by breaking due to fatigue, or by shakedown into a shape where they no longer deliver adequate power. Some features of the behaviour of mainsprings used as power sources are described and illustrated by examples. In particular, the importance of correct lubrication is emphasised. The primary loading on a mainspring is a bending moment, but sequential biaxial loading can occur, as can biaxial stresses due to distortion at end fittings. A fracture mechanics approach to some aspects of mainspring behaviour is promising.