Controversy surrounds the way in which the ANMC Competency Standards for Registered Nurses (the Standards) are understood and applied in the assessment of nurses' competence for practice. Adopted by the Australian Nursing Regulatory Authority in the early 1990s the Standards provide a description of the registered nurse on entry to practise, and have been used to determine both beginning level and ongoing competence to practise for over two decades. How nurses' interpret and make sense of the Standards particularly when making an assessment of competence is unknown. The study examines the deeper mechanisms which influence nurses' interpretation and application of the Standards in the context of assessment from a critical realist perspective. This perspective, together with a mixed methods methodology, enabled the re-conceptualisation and critique required to enhance understanding of the assessment of competence landscape. A two-phase exploratory mixed methods sequential design was employed. The questionnaire was used to gain prior insight into the context and to gain a general understanding of nurse's opinions around competence and its assessment in relation to the Standards. These findings were analysed to inform the qualitative phase. Interviews were then conducted with fifteen nurses comprising of; academics, clinical facilitators, preceptors and graduate nurses. Thematic analysis using a continual cross-comparative approach was used to explore how and why nurses operate at the interface between agency and structures in the context of competency assessment. Threads from both phases were drawn together and mixed at the data interpretation stage to achieve a cohesive set of findings. The thesis offers unique contributions into understanding the ways in which nurses create, negotiate and perpetuate the status quo of the Standards by focusing on the interface of nursing assessment processes and events, and their interactions with mechanisms and structures. Nurses create and operate in a state of tension created by their perception of the Standards as an important professional structure and the difficulty that they experience putting them into action in the context of practice. On the one hand, nurses support the Standards' as a mechanism to promote the collective identity of the nursing profession and accept the Standards as an integral part of the safety and quality agenda. On the other hand, nurses found the Standards difficult to operationalise for their key function of assessment. The thesis sheds new light on the way nurses interpret and use the Standards to assess competence; it is these subjectivities that are considered to be a critical new factor in the development of any future assessment framework. This is not only important new knowledge, but given the link between competence and the safety and quality agenda it has implications for the quality of patient care. The implications of the study findings for practice are therefore significant. Research into how competency assessment impacts on the quality of patient care needs to move beyond a quantitative, objective measure, to consider the generative potential of the socio-cultural context of practice. An urgent critical review of the Standards is needed to determine whether they continue to be a suitable assessment framework. The future success of any framework that assesses competence will be dependent on ensuring that both the structural forces and agentic subjectivities that shape nurses decision making are acknowledged and addressed.