This study employed the theory of planned behaviour (TPB) to investigate the factors underlying intentions and frequency of use of cannabis over a three-month period in a population of students (N=249). In addition, several hypotheses in relation to the TPB were investigated. The TPB provided good predictions of both intentions (R2=0.653; attitude, injunctive norms and perceived behavioural control significant) and behaviour (R2=0.711; intentions significant). Other norm measures (descriptive and moral norms) explained additional variance in intentions. (p<.01). In addition, habit strength and self-identity explained significant additional portions of the variance in intentions (p<.001), but not behaviour, over and above the TPB variables. Several interactions among these variables were also tested. Attitude moderated the impact of perceived behavioural control (PBC) on intentions (p<.001). Moral norms moderated the impact of attitudes on intentions (p<.001). Habit strength moderated the impact of self-identity on intentions (p<.001). PBC was found to moderate the impact of intentions on behaviour (p<.05). The findings are discussed in relation to how interaction effects further our understanding of the social processes by which variables are related in the TPB.