This thesis development of empirical. ABSTRACT , investigated play in two the ways: structure theoretical and and The theoretical section covers three areas. First, existing literature on animal play was reviewed to identify the basis for confusion in theories of animal play. Second, a discussion of the acquisition of the concept 'play' in humans, and its effect on our understanding of animal play revealed the appropriateness of the correct use of models and analogies in the study of play. Finally, the utility of Markov Chains, Hierarchical Cluster Analysis and the Grammatical Model in studying play, were assessed. In Chapter Six an investigation of "thB . nevelopment \.1\3: "'e.~ of social play and object 'predation'~ s owed no major ontogenetic differences. In both cases, a) there was some evidence for age changes in the composition of play/'predation': b) the majority of observations could be accounted for by a group of five 'dominant' behaviour patterns. Chapter Seven examined the function o~ 1'r~EM~p."en Mouth Play Face (OMPF) as a. play signal. Resurts indicated that the OMPF did not function speci~iCallY as a play signal in young ferrets and puppies. However, the situation was reversed for adult ferrets. There was also evidence that the OMPF was related to arousal in a way not exhibited by other play behaviour patterns. Chap~i~ml~!Sht investiga~~~bf~~ential S~~~hture in social ptay~ OBject 'predation~ an~ aggressi8~.~~esults showed great variability in tne sequences of 'oehaviour in all three contexts. All three behavioural contexts showed little variability in the identity of bout initiators and terminators. The 'grammar' constructed failed to provide a formal description of social play, aggression or object 'predation':' These results suggest that many assumptions made about the structure of play, especially with regard to other behaviours, are invalid. The study of play should proceed in terms of identifying the structural configuration of sequences of social play.