The theory of planned behaviour (TPB: Ajzen, 1988, 1991) was used as a framework to investigate homeless people's participation in outreach service programmes. In total, 104 homeless people from South Wales were interviewed using a schedule based on the TPB. Congruent with previous research on the TPB, attitude was the dominant predictor of behavioural intentions, and intention and perceived behavioural control were predictive of behaviour. Contrary to predictions, subjective norms also exerted a direct effect on behaviour. The discussion focuses on two issues: first the utility of social cognition models in explaining the relationship between demographic variables and behaviour in homelessness research; second, the direct effects of norms on behaviour and the extent to which work on social groups might usefully extend research on models such as the TPB to aid understanding of behaviour amongst stigmatized populations.