Description: Background: Smoking in people with mental health problems (MHPs) is an important public health concern as rates are two to three times higher than in the general population. While a strong evidence base exists to encourage and support smoking cessation in the wider population, there is limited evidence to guide the tailoring of interventions for people with MHPs, including minimal understanding of their needs. This paper presents findings from theoretically-driven formative research which explored the barriers and facilitators to smoking cessation in people with MHPs. The aim, guided by the MRC Framework for the development and evaluation of complex interventions, was to gather evidence to inform the design and content of smoking cessation interventions for this client group. Methods: Following a review of the empirical and theoretical literature, and taking a critical realist perspective, a qualitative approach was used to gather data from key stakeholders, including people with enduring MHPs (n = 27) and professionals who have regular contact with this client group (n = 54). Results: There was a strong social norm for smoking in participants with MHPs and most were heavily addicted to nicotine. They acknowledged that their physical health would improve if they stopped smoking and their disposable income would increase; however, more important was the expectation that, if they attempted to stop smoking, their anxiety levels would increase, they would lose an important coping resource, they would have given up something they found pleasurable and, most importantly, their mental health would deteriorate. Barriers to smoking cessation therefore outweighed potential facilitators and, as a consequence, impacted negatively on levels of motivation and self-efficacy. The potential for professionals to encourage cessation attempts was apparent; however, they often failed to raise the issue of smoking/cessation as they believed it would damage their relationship with clients. The professionals' own smoking status also appeared to influence their health promoting role. Conclusions: Many opportunities to encourage and support smoking cessation in people with MHPs are currently missed. The increased understanding provided by our study findings and literature review have been used to shape recommendations for the content of tailored smoking cessation interventions for this client group.