The purpose of this study is to explore the power dynamic when working with clients who use substances in a problematic way. The concept of power, including the meaning and nature, has been widely researched and debated. Similarly, numerous studies regarding therapeutic work with substance users are also available. However, there is a lack of material which links the two fields. An implicit power imbalance exists between therapist and client that is particularly relevant to this client group; and the egalitarian nature of person-centred counselling seeks to minimise this disparity. This study seeks to identify the nature of power within a therapeutic context and explore the impact of power dynamics when working with clients who use substances. Themes identified are (1) Diverging definitions of power; (2) Power dynamics within therapy; (3) The stance of person-centred counselling; (4) Issues when working with substance misuse; (5) The importance of disadvantageous external factors on the experience of personal power and (6) The efficacy of the person-centred approach in this field. Findings revealed that power can be experienced implicitly as well as explicitly, and that therapist awareness of the power imbalance is crucial. Furthermore, clients with problematic substance use frequently experience adversity on several levels and have an increased vulnerability to feelings of powerlessness. This study contributes to the research into the efficacy of person-centred counselling when working with this demographic.