This well-written and lively account of the principles of how motivational systems operate includes discussions of both theories and empirical results from individual systems. It reviews current experimental evidence on hunger, thirst, sex and other areas and argues that common factors must be emphasised as much as differences between the systems. The book summarises the theoretical principles that emerge: it shows where motivation theory and learning theory should come together, rather than diverge. Models with general predictive power are elaborated and related to the goal directed aspect of motivation. The book deals with motivation at all levels from the physiological to that of mathematical modelling and explains complex ideas lucidly. It complements other books in the Problems in the Behavioural Sciences Series including Hunger (le Magnen), Thirst (Rolls & Rolls) and Contemporary Animal Learning Theory (Dickson).