This paper proposes a theoretical model to explain trainee withdrawal and persistence within higher education (HE) based initial teacher training (ITT). Within ITT, attrition and persistence are under-researched and under-theorised, thus providing the rationale for this study. Within HE more broadly, a number of models have been proposed, most notably the seminal work of Tinto. However Tinto's Longitudinal Model of Departure was postulated for undergraduate academic courses, and has weaknesses in the study of trainees on professional courses, such as ITT. The proposed theoretical model is informed by interview and questionnaire evidence from a new case study of withdrawal and persistence in primary HE-based ITT. In this study, withdrawal and persistence were affected by factors across a range of domains: intra-personal, inter-personal, academic, professional, institutional and external. Tinto's model is evaluated against the empirical findings of the case study, allowing the proposal of a revised model which more effectively explains persistence and attrition in ITT. (Contains 3 tables and 4 figures.)