In this paper, we analyse the medical specialty training system in Spain (the so-called "residency system"). In order to do so, we a) summarize its historical evolution; b) describe the five major architectural pillars on which the system is currently based; c) analyse the special contract of the specialist-in-training; d) discuss the three major challenges for the medical specialist training future: the evolution and expansion of the residency system to other health professions, the issue of grouping specialties with a common core trunk and the continuity of the learning process; and e) draw four conclusions that may be relevant for those who are in the process of developing or revising their own medical specialization systems.