INCA (Intervention and Neuropsychology of Cannabis Abuse) is a study combining two research goals. One is to study efficacy of a short intervention the ‘Problem Profile Intervention’, which utilizes individual feedback from a ‘substance problem check-up’. Subjects will randomly receive the ‘Problem Profile Intervention’ or the WHO Brief Intervention, a brief intervention, which has been shown to be efficacious. Currently efficiency studies are under way. The WHO Brief Intervention has been translated into German by our group. In contrast to most other cannabis intervention studies the current study is focussed on cannabis abusers not so much cannabis-dependent subjects. A total of 160 subjects are to be recruited. Half of them will randomly receive either the Problem Profile Intervention or the WHO Brief Intervention (active control). A total of 25% of subjects from each group will be placed on a 3 months waiting list before receiving the intervention (inactive control). Intervention outcome will be assessed three months, six month and 12 months after the intervention. We hypothesize cannabis use to be reduced by 25% by the 'Problem Profile Intervention'. The second aim of the study is to study mechanism of substance abuse. Initial results from own studies and data from other groups point to executive cognitive functions as a subtle but important factor that can be compromised in cannabis users (Verdejo-Garcia et al., 2004; Garavan and Stout, 2005). Using an fMRI-study, we intend to examine a subgroup of our subjects on a specific aspect of executive functioning that has recently received major interest in drug research: Decision-making capability, or risk-taking behaviour, respectively (e.g. Ernst and Paulus, 2005).