Recent research has highlighted the potential importance of collective efficacy in enhancing understanding of group productivity within both mainstream and sport psychology, The aim of this thesis was, within a sport and physical activity setting, to progress beyond examining the collective efficacy - performance relationship and to focus upon examining the mechanisms through which Bandura (1982,1986,1997) predicts collective efficacy influences performance. Consistent with Bandura's proposals it was predicted that collective efficacy beliefs would influence group activity choice, goal selection, effort allocation, persistence levels, affective (including competitive state anxiety) reactions and team-referent attributions. A further research aim was to examine potential sources of collective efficacy. Thus, the role of team performance experiences and the attributions to these experiences as determinants of collective efficacy beliefs were examined. To achieve these aims five studies, three experimental studies and two field based surveys, were conducted. The experimental studies provided support for the prediction that an individual's collective efficacy beliefs are important determinants of that individual's stated choice of activity for their group, the goals they advocate that their group adopts, and the effort they allocate to the group task. Partial support for Weldon and Weingart's (1993) proposed relationship between collective efficacy and group goal commitment was also observed. However, no support was found for the predicted differences in persistence between individuals high and low in collective efficacy. The second and third of these studies indicated that performance information was an important source of collective efficacy. The field based surveys also provided support for aspects of Bandura's model of collective efficacy. The first of these indicated the existence of a small, negative relationship between an individual's collective efficacy beliefs and the level of cognitive anxiety experienced prior to competition and a moderate, positive relationship between an individual's collective efficacy beliefs and the level of positive affect experienced prior to competition. The second of the survey studies indicated that individuals with high collective efficacy used more controllable attributions than did those with low collective efficacy. Furthermore individuals' collective efficacy beliefs were observed to influence the team-referent attributions made after perceived success and failure of the team's performance. Specifically following perceived poor performances, high collective efficacy individuals used more external and unstable attributions than did those low in collective efficacy, whilst following good performances high collective efficacy individuals used more internal and stable attributions. This study also provided support for the role of controllable team-referent attributions in mediating the influence of performance attainments on collective efficacy beliefs with the use of controllable attributions leading to increases in collective efficacy following success and failure. Overall the research conducted provides support for a number of the mechanisms through which it is proposed that collective efficacy operates on performance, and for the role of performance attainments and team-referent attributions in determining collective efficacy. It also provides further evidence for the importance of collective efficacy to the understanding of group productivity and individuals who constitute groups.