This thesis examined some of the psychosocial determinants of performance. It is presented as a series of four interrelated research papers, which constitute the four main studies undertaken. These are preceded by a general introduction and succeeded by a general discussion. Study one concerned the issue of multidimensional performance assessment in sport. Specifically, it reported confirmatory factor analyses of a measurement instrument for tennis performance, first reported in Rees, lngledew and Hardy (1999). Generally, the results provided support for the seven-factor structure of the instrument. Study two concerned the assessment of social support in sport. Specifically, it reported confirmatory factor analyses of the Social Support Survey (SSS) (Richman, Rosenfeld, & Hardy, 1993), which has been increasingly used in sport. The results of a multitrait-multimethod analysis suggested that the structure of the SSS could be modelled. However, criticisms were levelled at the rationale behind the design, content and construction of the SSS. It was concluded that caution should be exercised in using this instrument in research and applied settings. Study three reported qualitative analyses of in-depth interviews with high-level athletes regarding their social support experiences. Grounded Theory Analysis indicated a four-dimensional model of social support, with specific quotes informing the construction of a questionnaire for study four. Study four represented the culmination of the preceding three studies. It examined the effects of stressors and social support upon performance. The refined performance measure and a measure of social support derived from study three were used, as well as sport-specific stressors. Stressors and social support dimensions were matched in order to examine the main and stress-buffering effects of social suppo1t upon performance. Moderated hierarchical regression analyses revealed significant main and stress-buffering effects of social support upon performance. Implications for future research and applied practice are derived from the four studies.