This thesis examined the development of advanced practice nursing in Thailand. The research focused on the perceptions of advanced practice nursing among Thai health care professionals and the factors affecting Thai nurses who are in the process of preparing to become advanced practice nurses (APNs). Although a large amount of literature about APN has been published, only one study has reported on the situation of the APN in Thailand. Other papers have been conducted with developing countries which employed different health services and nursing service systems. Various issues arose and were reported since the concept of APN was implemented in those countries, e.g. definition, qualifications, examination, registration, boundaries, incentive schemes and scope of practice. It was argued that nurses had encountered various obstacles during the preparation and transformation in becoming APN. Nevertheless, Thailand was only at the beginning of adopting this concept into the health service system. Therefore, this research aimed to investigate the development of APN in Thailand, which has never been explored. The study concentrated on the development of APN in nurses who are working in intensive care units (ICUs) because the model and the role of APN in this clinical area lack clarity in the published literature. Triangulation of two research methods was employed to address the research questions. The sample included staff nurses, head nurses, doctors and master degree students in an APN preparation programme. In the quantitative study, 226 questionnaires developed by the author were posted to 23 hospitals and 28 master degree students, yielding 156 usable responses. Qualitative data were collected from five focus group interviews with 28 master degree students using a semistructured interview schedule developed and piloted by the author. Factor analysis of quantitative data revealed five expected roles of the APN in ICU: extended roles, ensuring standards, patient management, nursing roles and academic roles. Regression analysis explained that there were significant relationships between being a head nurse and ensuring standards (P<O.5), age and nursing roles (P<O.5) and being a doctor and patient management (P<O.5). Being a head nurse, age and being a doctor appeared to be strong predictors of ensuring standards, nursing roles and patient management respectively. Qualitative data were analysed following Miles and Huberman and Strauss and Corbin's guidelines. Two central categories emerged: passage and process of the development of APN. Finally, a paradigm model was constructed, explaining the development of APN. Some concepts in the paradigm model were consistent with published literature, e.g. the readiness to change, transition and role theory. Other concepts were reflections of the unique Thai context, e.g. preparations for the development and anticipation of the role of APN. Thus, the development of APN in Thailand is a combed model. The thesis concludes with suggestions for the future research.