This article examines the mechanisms of ascription of responsibility in organizations, in a context of growing concerns about risk prevention and management. It is argued that the increasing appeal to professional knowledge and expertise, combined with a decline of trust in professional mechanisms of self-regulation, has led to a movement of ‘responsibilization’ of professionals within organizations. A central mechanism in this process is the increased demands for certified expertise addressed to professionals in organizations. This general argument is illustrated by fieldwork on the uses of psychiatric expertise in French prisons. The rise of the ‘new penology’ has put increased pressure on medical staff to assess prisoners and to deliver various sorts of ‘certificates’ for purposes that have often more to do with custody than care. The recent wave of suicides in French prisons has shown that, in case of unfortunate events, psychiatrists can be made personally responsible for the certificates they have produced and can be called into question by prison managers or judges. These ‘certified expertises’, located at the frontier of medical care and organizational categorization, will be used as a case study in order to shed light on the processes of ‘responsibilization’ of professionals in organizations.