The purpose of this thesis was to examine the range of prospective memory and executive function deficits in ecstasy/polydrug users and the role of these processes in accounting for the observed prospective memory performance deficits. Using a variety of laboratory and self-report measures of prospective memory and a self-report measure of executive function, ecstasy/polydrug users were tested in laboratory settings on measures of event and time-based, short and long term prospective memory as well as on a wide range of executive function components. It was found that ecstasy/polydrug users in relation to non-users experience more general prospective memory problems as ecstasy/polydrug-related deficits were evident on both time and event-based and short and long-term prospective memory. Ecstasy/polydrug users also demonstrated deficits on executive processes suggesting that recreational drug users are impaired in a broader range of executive function and ecstasy/polydrug-related deficits are not restricted to the three-model component of executive function. It was also found that executive dysfunction is associated with poorer time-based prospective memory and perhaps some of the drug related prospective memory deficits are mediated by drug related executive function impairment. Finally, although few prospective memory or executive function performance deficits were evident among cannabis-only users a trend was evident in all investigations; ecstasy/polydrug users perform the worst, cannabis-only users at intermediate levels and drug-naïve perform the best. The most striking finding of the present thesis was that the recreational use of cocaine was associated with PM deficits; an association that consistently emerged in all studies of PM performance. The outcomes of the present thesis provide a fruitful direction for future research.