Among the many roles clinicians are expected to perform is that of educator of junior colleagues. However, most clinicians have received little or no developmental instruction for this role. Furthermore, the nature of medical education is changing, and the skills required of new graduates are being refocused. This series of three articles presents a guide to some of the philosophical and educational issues at the heart of current changes in medical education. As well as developing an argument for making practice-based clinical teaching student-centred and problem-orientated, suggestions for planning and implementing teaching which utilise these approaches are outlined. In this first article the changing nature of clinical teaching is discussed and the educational principles of problem-based and student-centred learning are defined. The second article looks at the steps involved in planning a clinical teaching session. The third and final article looks at the development and implementation of teaching sessions.