The aim of the thesis is to study the role of instruction in the interconnection of instruction-learning-development. The thesis consists of six empirical papers and a summing-up and perspectivizing introductory paper. Five of the empirical studies concern so called heart conferences, clinical diagnostic meetings, which at the time of my study, 1995-1996, were arranged as telemediated conferences between a sub-team of surgeons and radiologists in a university clinic, and a sub-team of cardiologists and radiologists in a regional hospital. The outcome of the coronary diagnostic work in the heart conferences was patient diagnoses and decided-upon treatment (surgery, balloon dilatation, or conservative treatment). The sixth empirical study, conducted in the autumn 2000, investigates the design and redesign of a central artifact used in the heart conference, ?the angio film,? produced in the angio lab. A recurrent theme in the empirical papers is whether artifacts might be instructive and, if so, in what ways. The introductory paper is a hybrid between an ordinary summing-up paper of the findings in the empirical studies, and a perspectivizing presentation of activity-theoretical approaches to instruction, learning and development, elaborating on three basic aspects (learning as a collaborative phenomenon, the instructiveness of artifacts, and the relation between learning and development on an individual level, but primarily on an activity level). In conclusion, my study outlines an approach to learning based on new perspectives on instruction. / Studier av läkares co-coaching av varandra som ett led i deras samarbete rörande kranskärlsdiagnostiskt arbete. Artefaktanvändning, lärande och versamhetsutveckling.