The first year of practice after medical school is considered to be an essential part of becoming a medical practitioner in Australia. Previous qualitative investigations have investigated a number of significant aspects of this early stage of professional development. This qualitative study explores experiences and developing professional identities during internship. Thirty interns and six intern supervisors were interviewed from three different Australian states. Grounded theory techniques were used to develop three key themes: internship-as-participation, internship-as-progression, and conflicts, parallels, disturbances and outliers. Key findings were: the important balance between support from colleagues and development through taking independent responsibility; and the strength of the view of internship as part of a 'natural progression', an inevitable evolution through the stages of medical training.