This thesis describes a series of studies that examine how nurses who work with people with intellectual disability make clinical decisions in practice. Current research offers evidence and guidance to support clinical decision making for nurses working within mainstream healthcare services. However, the available reviews of intellectual disability nursing research have identified that evidence to support intellectual disability nursing practice is sparse. Consequently, little is known about how and what decisions intellectual disability nurses make in practice; and the types and sources of evidence they use to support their practice. In the context of intellectual disability nursing and in light of a review of the literature (Chapter 1 and 2), three studies were conducted. The aim of this thesis was to use mixed methods (Chapter 3) to explore the process of clinical decision making in intellectual disability nursing. In Study 1 (Chapter 4), Interpretative Phenomenological Analysis was used to explore how twelve intellectual disability nurses made decisions in practice and how their decisions were influenced by evidence. Four key themes emerged: 1) getting to know the person, 2) working as a team, 3) evidence to support decision making, and 4) understanding of evidence based practice. An all Wales survey (Study 2) of nurses employed by the National Health Service intellectual disability nursing service in Wales was conducted. The survey determined the types and sources of evidence nurses used to make decisions in practice; further identifying the demographic and cognitive characteristics that influenced the use of evidence (see Chapter 5). Study 3 explored how individuals with intellectual disability perceive the role of intellectual disability nurses (Chapter 6). Seven participants engaged in semi structured interviews and the analysis of the data was guided by Foucault’s concepts of dividing practice, scientific classification and subjectification. Two discursive themes emerged from the analysis: (1) Inclusion: empowering and supportive healthcare practice; and (2) Exclusion: challenges and practices that resist healthcare involvement. The integrated findings are discussed in relation to their contribution to the literature, methodological challenges, implications for intellectual disability nursing practice, education and recommendations for future research (Chapter 7).