Background: Many speech sound disorder (SSD) interventions with a long-term evidence base are 'new' to clinical practice, and the role of services in supporting or constraining capacity for practice change is underexplored. Innovations from implementation science may offer solutions to this research-practice gap but have not previously been applied to SSD. Aim: To explain variation in speech and language therapy service capacity to implement new SSD interventions. Methods & Procedures: We conducted an intensive, case-based qualitative study with 42 speech and language therapists (SLTs) in three NHS services (n = 39) and private practice (n = 3) in Scotland. We explored therapists' diverse experiences of SSD practice change through individual interviews (n = 28) or self-generated paired (n = 2) or focus groups (n = 3). A theoretical framework (Normalization Process Theory) helped us understand how the service context contributed to the way therapists engaged with different practice changes. Outcomes & Results: We identified six types ('cases') of practice change, two of which involved the new SSD interventions. We focus on these two cases ('Transforming' and 'Venturing') and use Normalization Process Theory's Cognitive participation construct to explain implementation (or not) of new SSD interventions in routine practice. Therapists were becoming aware of the new interventions through knowledge brokers, professional networks and an intervention database. In the Transforming case, new SSD interventions for selected children were becoming part of local routine practice. Transforming was the result of a favourable service structure, a sustained and supported 'push' that made implementation of the new interventions a service priority, and considerable collective time to think about doing it. 'Venturing' happened where the new SSD interventions were not a service priority. It involved individual or informal groups of therapists trying out or using one or more of the new interventions with selected children within the constraints of their service context. Conclusions & implications: New, evidence-based SSD interventions may be challenging to implement in routine practice because they have in common a need for therapists who understand applied linguistics and can be flexible with service delivery. Appreciating what it really takes to do routine intervention differently is vital for managers and services who have to make decisions about priorities for implementation, along with realistic plans for resourcing and supporting it.