In the United Kingdom, policy and guidance changes regarding the role of dental therapists (DTs) were implemented in recent years with a view to changing dental care to a more preventive-focussed, teamwork approach. However, success in the adoption of this model of working has been varied.Adopting a realist approach, our aim was, to examine the use of DTs in general dental practices in Wales, exploring what works, why, how and in what circumstances.The research comprised two stages. (a) A structured literature search, dual-coding papers for high-level factors describing the conditions or context(s) under which the mechanisms operated to produce outcomes. From this, we derived theories about how skill-mix operates in the general dental service. (b) Six case studies of general dental practices (three with a dental therapist/three without a dental therapist) employing a range of skill-mix models incorporating semi-structured interviews with all team members. We used the case studies/interviews to explore and refine the theories derived from the literature.Eighty-four papers were coded. From this coding, we identified seven theories which reflected factors influencing general dental practices within three broad contexts: the dental practice as a business, as a healthcare provider and as a workplace. We tested these theories in interviews with 38 dental team members across the six care studies. As a result, we amended five of the theories.Our analysis provides theory about outcomes that DTs may facilitate and the mechanisms that may assist the work of DTs within different contexts of general dental practice.