This paper provides a critical review of the litera-ture on severe mental illness (SMI) and type 2diabetes and presents an argument for the develop-ment of theory-based interventions tailored speciﬁ-cally for people with these co-existing conditions.Individuals with SMI are at an increased risk ofphysical health problems, with a high prevalence oftype 2 diabetes among this population. Yet, it isestimated that up to 70% of cases of diabetesamong people with SMI are undiagnosed (Holt &Mitchell 2015), which has many clinical implica-tions for the individuals themselves and the healthcare professionals involved in their care. Changesin lifestyle can signiﬁcantly reduce the risk of devel-oping diabetes in the ﬁrst place and also reduce therisks of complications from the condition. How-ever, lifestyle changes relating to diet, exercise andsmoking can be particularly challenging for peoplewith SMI. There is good evidence that educatingpeople with diabetes about the disease in generaland behaviour change to manage the condition canhave a positive impact on clinical outcomes in thegeneral population (Steinsbekk et al. 2012). How-ever, less is known regarding the effectiveness andappropriateness of existing diabetes self-manage-ment programmes for people with SMI. This essaywill present an argument for the use of theory andresearch evidence to strengthen the developmentand implementation of new interventions speciﬁ-cally for people with SMI and type 2 diabetes.