BackgroundThe implementation of new and complex interventions in mental health settings can be challenging. This paper explores the use of a Theory of Change (ToC) for intervention design and evaluation to increase the likelihood of complex interventions being effective, sustainable, and scalable. Our intervention was developed to enhance the quality of psychological interventions delivered by telephone in primary care mental health services.MethodsA ToC represents how our designed quality improvement intervention targeting changes at service, practitioner, and patient levels was expected to improve engagement in, and the quality of, telephone-delivered psychological therapies. The intervention was evaluated following implementation in a feasibility study within three NHS Talking Therapies services through a qualitative research design incorporating semi-structured interviews and a focus group with key stakeholders (patients, practitioners, and service leads) (N= 15). Data were analysed using the Consolidated Framework for Implementation Research (CFIR) and the ToC was examined and modified accordingly following the findings.ResultsCFIR analysis highlighted a set of challenges encountered during the implementation of our service quality improvement telephone intervention that appeared to have weakened the contribution to the change mechanisms set out by the initial ToC. Findings informed changes to the intervention and refinement of the ToC and are expected to increase the likelihood of successful future implementation in a randomised controlled trial.ConclusionsFour key recommendations that could help to optimise implementation of a complex intervention involving different key stakeholder groups in any setting were identified. These include: 1-developing a good understanding of the intervention and its value among those receiving the intervention; 2-maximising engagement from key stakeholders; 3-ensuring clear planning and communication of implementation goals; and 4-encouraging the use of strategies to monitor implementation progress.