The notions of purpose, goal, end and function are used (and abused) in descriptions of a very wide range of human, animal and machine behaviour. Andrew Woodfield provides here a unified account of such teleological descriptions and explanations, their varieties, their logical structure and their proper uses. He concentrates his argument on the concepts of 'goal-directed behaviour' and 'natural function', and combines original philosophical criticism with a meticulous, detailed survey of the main competing theories in this diffuse and difficult field.