The Hybrid Identities Project (HIP) investigates the professional role and identities of Approved Mental Health Professionals (AMHPs) as hybrid professionals. Hybrid professionals are professionals of mixed origin who operate within broad knowledge areas with competence developed outside of their main area of expertise. AMHPs are hybrid in several interrelated ways; in the varied professional specialisms that became generic for the AMHP role when it was extended from the previously exclusive specialist Approved Social Work role, and in the areas of expertise enacted through roles, boundary spanning and professional perspectives that encompass legal, social, therapeutic and psychiatric jurisdictions of practice. This means that AMHPs have complex and negotiated professional roles; therefore, hybridity within identities is a valid site in which to explore the types of hybridisation visible within AMHP professional roles and identities. The methodology is a case study of professional identities located in the stories of ten AMHPs from social work, nurse and occupational therapy backgrounds. Data is generated through thirty topic-based interviews conducted in three stages over a period of two years. Each interview explores specific but interrelated areas of professional roles and identities, comprising participants' espoused knowledge, skills and values, influences on career choices and professional hybridisation. The research is conceptualised by drawing on social and professional identity theorisations, concepts of hybridity and dialogism. Illuminative participant data focusing on hybrid 'perspectives in use', hybrid roles and boundary spanning are presented from social work, nurse and occupational therapy primary professional backgrounds. These interrelated dimensions are expressions and enactments of hybrid professionals' identities, as they reveal how participants author the world through language that is 'borrowed' from other professionals and how participants navigate boundaries between social, healthcare and external organisations, indicating epistemological hybridity. Boundaries are negotiated that cross-cut traditional jurisdictions of other specialist professional roles claimed, illuminated through participants' everyday practices to reveal professional identities as hybridised across all primary professionalities within the sample. This is followed by a detailed thematic analysis of professional hybridity that conceptualises AMHP professional identities as hybridised and dialogically negotiated. Professional identities are negotiated within the employing organisation's culture but interplay with working relationships amongst other professionals within Mental Health Act work and with broader professional, systemic and socio-political contexts in contemporary life, creating 'ironic dialogism'. In investigating AMHPs as hybrid professionals negotiating dialogical identities, the HIP contributes to knowledge of AMHP multi-professional roles and identities, professional hybridity, dialogical identities and the sociology of professions. It illuminates AMHPs' biographies as they are enacted and located in the intersubjective professional realities of everyday life, contributing sociologically to broader understanding of professional hybridity in related professional groups and an understanding of boundary spanning as a site of (re)organisation for AMHP professional identities.