The key characteristics of advanced nursing practice have been a subject of international debate over the past decade. To address this debate, a grounded theory study was undertaken by one of the authors which sought to identify the key characteristics of advanced nursing practice in adult critical care. The outcome of the main study was a theory of legitimate influence in which enhancing patient stay and improving patient outcome represented the dual purpose of advanced nursing practice in critical care. Fundamental to these factors is strategic activity. This encompasses improving patient care, facilitating continuity of care and engaging in patient education. The outcome of these strategic activities can be evaluated through evidence of eased transition across complex hospital networks, patient satisfaction and enabling of independence. The findings reflect a change in the focus and delivery of care to the critically ill and their relatives by nurses practising at an advanced level. In the second paper of this series, the intervening conditions that affect the expression of legitimate influence will be discussed.