Abstract: The introduction of nurse practitioner registration into New Zealand in 2001 was heralded as a move that would open up a wealth of opportunities for registered nurses to extend their practice into more independent roles and to provide a client-centred health service. It was also seen as a way to retain experienced registered nurses in the clinical practice area by providing a credible clinical career pathway. If nurse practitioner's are to meet these expectations, then, the author suggests, it is important to understand the processes that encourage or discourage nurses from advancing their practice. One of the early scopes of practice to be introduced was nurse practitioner with an endorsement in aged care scope of practice. Grounded theory was the method used to generate an explanation of how registered nurses working in aged care were preparing for the introduction of nurse practitioner roles. An analysis of early data highlighted codes around registered nurses in aged care extending and advancing their practice rather than preparing specifically for the nurse practitioner role. The research question for this study was: 'How are registered nurses in aged care advancing their nursing practice?' Semi-structured interviews were used to collect data from ten experienced registered nurses working in aged care clinical practice settings ranging from secondary hospital facilities, to community settings and residential care villages. Dimensional analysis of the data eventually generated three major conceptual categories: 'ownership of nursing', 'extending practice', and 'moving out of a comfort zone'. Of these, 'ownership of nursing' was identified as the core construct that linked the other categories together. The substantive theory that explains how registered nurses in aged care advance their clinical practice is 'developing autonomous ownership'. Nurses who develop autonomous ownership of nursing are more likely than other nurses to move out of a current comfort zone and advance their practice into more independent roles that suit their autonomous ownership of nursing. This study identified important contextual factors and conditions that support the development of an autonomous ownership of nursing and that subsequently facilitate advancing nursing practice. These include creating supportive environments, organisational commitment to advanced nursing practice roles, visible nursing leadership, congruence between organisational and nursing philosophies, interdisciplinary collaboration and participating in postgraduate education. The author suggests that the significance of this study is that it generated a theory about the processes that encourage or discourage nurses from preparing for, and progressing into, advanced nursing practice roles such as nurse practitioner.