The aim of this study was to explore the perceived impact of advanced practice nurses in promoting evidence-based practice amongst frontline nurses. A collective instrumental case study was undertaken involving five extended case studies and eighteen short case studies in a range of hospital and primary care settings across seven Strategic Health Authorities in England. The study participants were a purposive sample of 23 advanced practice nurses selected to represent a range of settings, clinical specialities, organisational responsibilities and ways of working. In-depth interviews were undertaken with the advanced practice nurse and up to 10 interviews with health care professionals with whom they worked. For the extended case studies, non-participant observation and follow-up interviews were also undertaken. Data analysis drew on the principles of the Framework approach.From the perspectives of the participants, these advanced practice nurses enhanced the ability of frontline nurses to provide evidence-based care. They improved the competence, knowledge and skills of frontline nurses and empowered them to deliver care which they considered to be safer, holistic, more timely and of a higher standard. This is likely to have a positive effect on patient outcomes and on patient experience. However, this impact is inherently hard to capture.