Self-determination theory (SDT; Deci & Ryan, 1985, 2000) is an approach to understanding human motivation which holds that the satisfaction of certain innate psychological needs is the basis for self-motivation, psychological growth, and optimal well-being. Cognitive evaluation theory (CET: Deci & Ryan, 1985), a sub-theory within SDT, provides further detail regarding how events relevant to the initiation and regulation of behaviour can impact upon psychological need satisfaction and subsequent well-being. Events are posited to have one of three aspects, informational, controlling, or amotivational. Drawing on this theoretical framework, the present thesis adopts the position that self-talk represents an internal regulatory event that can be experienced as informational or controlling, with subsequent differential consequences for behavioural and affective outcomes. Consisting of a general introduction, five empirical chapters, and a general discussion, this thesis had three main aims. First, to extend self-talk research by examining its antecedents and effects in the context of a contemporary motivational theory. Second, to test an SOT-based model in which self-talk is a component of athletes' experience of the motivational environment in sport. The final aim of this thesis was to develop an understanding of athletes' behaviour in the training environment and the impact of self-talk on psychological need satisfaction and training behaviours. In order to achieve these aims, the effects of autonomy-supportive versus controlling environments on individuals' self-talk were examined, measures of informational and controlling self-talk and athlete training behaviours were developed, the relationships between the two types of self-talk and affect were explored, and the relationships between psychological need support, self-talk, need satisfaction and behavioural outcomes were investigated in a high performance sports setting. Taken together, this main findings of this series of five studies were as follows: (1) evidence was provided that self-talk can be meaningfully differentiated into informational and controlling components; (2) the degree to which the environment supports basic needs is related to the content and functional significance of self-talk; (3) the functional significance of self-talk is associated with a number of affective and behavioural outcomes; specifically, informational self-talk is associated with more desirable outcomes; (4) athletes' need satisfaction is predictive of their training behaviours; and (5) that examining the antecedents and consequences of how one experiences one's self-talk (i.e., its functional significance) in the context of SDT appears promising. This thesis has made contributions to motivation oriented research by providing further evidence to support the propositions of SDT regarding the benefits of need supportive environments for optimal motivational, affective, and behavioural outcomes. For the first time positive associations were identified between needs support, need satisfaction, and athletes' training behaviours. Furthermore, the crucial role of self-talk in the motivational experience was highlighted. Findings regarding the functional significance of self-talk have important theoretical and applied implications, not least highlighting the role of how individuals experience their self-talk in determining affective and behavioural response to the social environment. Further research which builds on this series of studies will lead to a greater understanding of how self-talk is related to motivational processes and human behaviour.