School-leaving for pupils with long-term speech, language, swallowing or communication difficulties requires careful management. Speech and language therapists (SLTs) support communication, secure assistive technology and manage swallowing difficulties post-school. UK SLTs are employed by health services, with child SLT teams based in schools. School-leaving entails transition from child- to adult-services. Little is known about the process, or how SLTs develop co-working across managerial boundaries. A qualitative study within one health board employing separately managed child and adult SLT teams interviewed SLTs and analysed their views on successful and less successful school-leaver transitions. A critical incident approach elicited views on transitions that "stuck in the mind," rather than typical instances, identifying supportive and risky co-working factors. Interviews were recorded, transcribed, checked and thematically analysed. Three linked overarching themes emerged: SLT team remits and properties; communication and information exchange across SLT teams, and outside influences on teams. These applied to successful and less successful transitions, suggesting robust constructs along which SLTs evaluated transitions. Risk factors included unclear provision, pupils' earlier discharge by child SLTs affecting referral at school-leaving, and practical issues in accessing notes. SLTs used existing social-capital relationships to facilitate transitions. Implications for practice and ways of improving transitions are discussed.