The overall aim of this PhD thesis was to conduct a comprehensive investigation of decision-making impairment in long term opiate users, using three studies. The first study aimed to determine the extent of the decision-making impairment and to establish whether other co-morbid factors impacted on the severity of this deficit. Using meta-analysis, the results indicated that opiate use is associated with relatively severe decision-making impairment, and that co-morbid factors, such as head injury and poly-substance dependence did not significantly change the magnitude of the impairment. Furthermore, the decision-making impairment in opiate users was not mitigated by abstinence, and the duration of opiate use and the duration of abstinence did not have a significant impact on size of the impairment. The second study analysed whether the somatic marker hypothesis, an emotion-based model of decision-making, could provide an explanation for the decision-making impairment in opiate users. This empirical study found that, although decision-making was impaired in a group of long term opiate users relative to a group of healthy controls, this impairment was not due to reduced emotional responsiveness, nor an inability to form anticipatory warning signals (i.e., somatic markers), as measured by the skin conductance response. Notably, stronger somatic responses when contemplating making disadvantageous choices were associated with worse decision-making in opiate users, which does not support the predictions of the somatic marker model of decision-making. Finally, the third study analysed decision-making under conditions of risk, to determine whether the impairment in opiate users was restricted to certain types of decision-making. This empirical study found that opiate users, although impaired in decision-making under conditions of ambiguity, were not impaired on decision making tasks involving calculable risk, relative to healthy controls. This study also demonstrated that opiate users' decisions were not driven by an increased responsiveness to reward. Together, the results of this thesis suggest that opiate users are particularly impaired in situations of decision-making under ambiguity, but not risk, and this is not due to impairment in emotional processing. This has implications for the treatment of opiate users, who may need additional training to appropriately utilise physiological signals to make adaptive decisions. The results of this thesis may therefore be used to inform treatment practice to better support opiate users during ambiguous decision-making situations in daily life.