RESUMEN
(1) Background: Identifying differences in the competencies of different areas of nursing is a crucial aspect for determining the scope of practice. This would facilitate the creation of a formal structure for clinical practice in advanced and specialised services. The aims of this study are to analyse the distribution of advanced competencies in registered, specialist and advanced practice nurses in Spain, and to determine the level of complexity of the patients attended by these nurses. (2) Methods: A cross-sectional study was developed on registered, specialist and advanced practice nurses, all of whom completed an online survey on their perceived level of advanced competencies and their professional characteristics. (3) Results: In total, 1270 nurses completed the survey. Advanced practice nurses recorded the highest self-perceived level of competency, especially for the dimensions of evidence-based practice, autonomy, leadership and care management. (4) Conclusions: Among registered, specialist and advanced practice nurses, there are significant differences in the level of self-perceived competencies. Patients attended by advanced practice nurses presented the highest levels of complexity. Understanding these differences could facilitate the creation of a regulatory framework for clinical practice in advanced and specialized services.