Cooperation in medicine, as in all group activity, is a key problem both for the actors involved and for sociologists. Classical studies in the sociology of health and medicine are moreover of little help in conceptualizing this problem. While arguing against the idea that affiliation with an institutional structure – such as a part of a professional segment or an area of expertise – determines relations between actors, our analysis seeks to move beyond the description of what is presented as being purely contingent. To understand phenomena related to cooperation and conflict, it is possible and worthwhile to distinguish between professionals according to their position in the therapeutic chain and their form of involvement in the strategy of care. Cooperation is based, in particular, on the match between “acquisitive” professionals who feel it is their duty to coordinate medical care and follow-up on a patient throughout the healing process, and other professionals who see their intervention as being limited to one phase in the therapeutic itinerary.