Professional structure and its relations to certain social fields is an underdeveloped topic of research. A common understatement is that new eras require new professions, that is, when fields change the professional structure also changes. In some cases, the statement might be correct, but this common-sense understanding has little theoretical or empirical support. It ignores the fact that professional structures sometimes actually remain unchanged for long periods, even though the related fields have developed, or vice versa. Nor does it take into account that the knowledge base of the professions can be modified to suit new requirements. Moreover, it seems to underestimate the opportunities for professions to take part actively in field definition. In this paper, a conceptual framework is proposed to enable an analysis of professional structures and their relations to social fields. The professional structure is defined as the composition of professional groups and the power relations between them. The social field is defined as an institutionalized power area comprising actors and institutions struggling for influence. Swedish urban planning offers an excellent case for illustrating the theoretical and methodological findings in the paper. It is concluded that the field of urban planning has changed in some respects, but not fundamentally. The professional structure, on the other hand, has shifted dramatically. The variation in jurisdictional claims and influence can explain why the structure has developed in the ways it has.