Impulsivity is a behaviour that has received several definitions. The most recent and widely accepted definition hypothesises that it is influenced by several separate behaviours including self-control, risk-taking and inhibitory control. Drug abusers and pathological gamblers have been found to exhibit less self-control than healthy controls which may be linked to their focus on short term, potentially damaging rewards, (e.g. positive effects of drug taking) rather than longer term rewards (e.g. better health) thus exacerbating their addiction. The links between risk-taking and addiction are not well understood as the few studies in this area have found contrasting results. This project was designed to explore self-control and risk-taking in pathological gamblers and substance abusers. Non-pathological gamblers were also recruited to investigate behaviour in a non-addicted sample. Novel delay discounting and probability discounting tasks were created which directly measured choice behaviour requiring self-control (or tolerance of delay) or assessment of risk. The tasks were designed to provide realistic consequences for every choice thus aiming to mimic real-world decision-making situations. These tasks also explored discounting behaviour when given real versus hypothetical monetary reward. Previous research has found contrasting results as to whether giving real reward in a delay discounting task significantly alters choice behaviour. In addition to analysis of discounting behaviour, imaging tasks were also created to explore brain areas involved in self-control, risk-taking, inhibitory control, and gambling urges. Differences in activity between the groups were assessed in order to discover any abnormalities. The results from this project have uncovered new information concerning everyday decision-making, the behaviour and neurology of behaviours affecting impulsivity and addiction. The results from the project also have wide ramifications for the validity of methodologies utilised in decision-making research.