This research assessed the cognitive functioning of several different drug and alcohol samples, with a specific focus on individuals in methadone maintenance treatment. Additionally, this research assessed the relation between cognitive functioning and adherence to treatment in these populations. This is a link that has been suggested by research in this area previously. The present research consisted of five experiments. Experiment 1 assessed the application, within New Zealand, of the normative data for a number of psychometric tests with a non- substance using sample of New Zealand university students. The results from this study showed that the scores for the university sample were generally similar to the normative means, suggesting that the existing normative data could be used in New Zealand. Experiment 2 assessed the cognitive functioning of university students who reported regular use of alcohol and/or cannabis. The results from this study showed that the frequency of alcohol use was positively correlated with problem solving ability, while frequency of cannabis use was positively correlated with memory functioning. Cannabis use was also associated with poorer problem solving ability. These findings showed that casual use of alcohol and/or cannabis was associated with poorer functioning on some psychometric tests, and that these tests were sensitive to the effects of substance use. Experiment 3 assessed the cognitive functioning of a sample of individuals in methadone maintenance treatment in relation to their adherence to treatment. The results from this study showed deficits in memory, divided attention and cognitive flexibility, and poor mathematical ability when compared to the normative data. Treatment adherence was found to be associated with lower levels of treatment satisfaction, and findings also suggested a possible relation between poor treatment adherence and better scores on the psychometric test scores. Due to difficulties in recruiting participants, Experiment 4 assessed the cognitive functioning of a second sample of individuals in methadone maintenance in relation to treatment adherence. This sample completed a reduced battery of psychometric tests, and results showed deficits in memory, attention, and problem solving abilities compared to the normative data. Treatment adherence was not found to be associated with lower levels of treatment satisfaction in this sample. A possible relation between poor treatment adherence and better psychometric test scores was identified for this sample also, although the relation was between different test scores than those in Experiment 3. To assess whether the obtained results for Experiment 3 and Experiment 4 were specific to methadone, Experiment 5 assessed the cognitive functioning of several different drug and alcohol samples in relation to treatment adherence. The study examined the cognitive functioning of alcohol, drug and alcohol, and other opiate users, and showed that all samples had difficulty recalling information in a visual format, that a large percentage showed impairments in divided attention and cognitive flexibility, and that the drug and alcohol, and other opiate samples had deficits in problem solving ability. Treatment adherence in this study was assessed for the alcohol sample, with results showing a relation between poor treatment adherence and verbal memory ability, and divided attention and cognitive flexibility. Overall, the cognitive deficits found in the samples for each experiment were not related to treatment adherence as measured in this research. However, results for the alcohol sample suggested that there may be a link between poor adherence and impairments in verbal memory and divided attention. The findings from this research suggest that cognitive deficits and treatment adherence are not related, but this finding may be the result of limitations in this research (i.e., recruitment difficulties, adherence measures used). The clinical and research implications of the results of the research are discussed. In particular, recommendations for treatment services dealing with the drug and alcohol population are provided.