The Anglo-American institution of the profession is mutating: we propose to analyse this mutation as a contested diffusion process that spreads new organizing practices among professionals. We offer an integrated account of the roles played in this diffusion/mutation process by facilitating and impeding factors at three levels: individual professionals (their autonomy, expertise, values, identities, and ties), professional organizations (their strategies, structures, cultures, skills, and systems), and the broader institutional field (professional associations, accountability demands, and competition). At the occupational and organizational level, we show how the distinctive and evolving features of professionalism moderate the mechanisms found in prior research on diffusion in other, non-professional settings; and at the field level, we show how field-level forces moderate the impact of professionalism on these diffusion dynamics. Changes at each of these levels interact with changes at the others, with influences flowing both downward and upward. We ground and illustrate this theoretical synthesis with evidence from the case of clinical guidelines as carriers of institutional change in the medical profession.