It is argued that a classification of play which can be systematically related to motivation and function is a prerequisite for empirical studies. 'Arousal' theories have come closest to providing a framework for understanding motivation and function, but the term 'arousal' itself requires careful definition. Definitions of the terms 'play' and 'arousal' are made after review of relevant literature in Chapters 1 and 2. The behavioural sequences which distinguish different kinds of play axe shown to be amenable to quantitative analysis which confirms qualitative behavioural distinctions (Chapters 3 and 4), and to have different functions: ludic play is innovative, whereas exploratory play is more akin to a learning experience (Chapter 5). Chapters 6,7 and 8 draw attention to analogies between manipulative and symbolic play, and experimental investigations explore the relationship between symbolic play and cognitive abilities. No evidence is found for a unique role of symbolic play in cognitive development. The validity of arousal theories is specifically tested in three studies reported in Chapter 9. Theoretical predictions suggest that the motivational antecedents of play and day-dreaming will be similar: the physiological correlates of play and daydreaming are shown to be similar, and to reflect lower states of arousal than are found during exploration and problem-solving. Finally, a model of play is proposed which attempts to integrate the experimental findings within the terms of motivational systems theory. This permits a classification of play which can be directly related to functional outcomes