Abstract Background: Speech-language interventions are often complex in nature, involving multiple observations, variable outcomes and individualization in treatment delivery. The accepted procedure associated with randomized controlled trials (RCT) of such complex interventions is to develop and implement a manual of intervention in order that reliable treatment delivery can be achieved. Aims: To present the rationale, structure and content of an intensive manualized intervention as implemented within an RCT for children with complex pragmatic and social communication needs; to investigate factors associated with implementation in a mainstream school environment; and to determine treatment fidelity. Methods & Procedures: The manualized SCIP intervention, including procedures for developing individualized treatment plans, was developed and then implemented within an RCT with 57 school-aged children with complex pragmatic communication needs (CwPLI). The paper describes the delivery protocol, staffing requirements, and content and structure of the intervention. A mapping procedure for individualization of intervention and the implemented components of intervention are presented. The findings from a school-therapy alliance checklist for recording factors affecting implementation in a school context are also reported. Treatment fidelity was carried out using measures of delivered versus planned treatment content and quality of therapy. Outcomes & Results: The manual was effective at detailing intervention procedures and allowing for development of individualized treatment plans whilst maintaining satisfactory treatment fidelity. Treatment planning and delivery required continuous specialist speech and language therapist input with assistants needing substantive training and supervision. Key components of intervention for CwPLI were therapies aimed at improving conversation skills, narrative construction, comprehension monitoring, understanding of social cues and metapragmatic awareness. The school-therapy alliance checklist indicated high rates of therapist-education staff and therapist-parent liaison. Parents were nearly always involved in treatment planning though only half-attended therapy sessions. Learning support provision in schools for participating children was highly variable. Conclusions & Implications: Predetermining theoretical background, structure, and content in a treatment manual contributed towards the quality and reliability of intervention within the context of an RCT. The implementation of SCIP intervention in a broader clinical context is discussed with reference to staff expertise, the therapeutic process and the essential ingredients of social communication intervention. © 2012 Royal College of Speech and Language Therapists.