Healthcare professionals such as speech and language therapists are expected to change their practice throughout their career. However, from a practice perspective, there is a lack of knowledge around what practice change is, what it really takes, and why there are different trajectories. Consequently, therapists, managers and commissioners lack empirical evidence on which to base decisions about enabling practice change. In addition, intervention researchers lack basic sociological research around implementation that could inform their research designs, reporting and impact. This case-based sociological inquiry, underpinned by critical realist assumptions, was designed to address this knowledge gap. It includes a two-stage qualitative synthesis of 53 (then 16) studies where speech and language therapists explained the work of their practice in depth, and a primary qualitative study focused on one professional jurisdiction, children with speech sound difficulties (SSD). Forty two speech and language therapists from three NHS areas and independent practice in Scotland participated in individual interviews or self-organised pairs or focus groups to discuss in depth how and why they had changed their practice with these children. A variety of comparative methods were used to detail, understand and explain this particular aspect of the social world. The resulting theory of SSD practice change comprises six configured cases of practice change (Transforming; Redistributing; Venturing; Personalising; Delegating; Refining) emerging from an evolving and modifiable practice context. The work that had happened across four key aspects of this context (Intervention; Candidacy; Caseload; Service) explained what made each case possible, and how practice had come to be one way rather than another. Among its practical applications, the theory could help services plan more realistic practice change. In addition, the inductively developed layered model of SSD intervention change has the potential to contribute to speech and language therapy education as well as methodological discussions around complex interventions.