Abstract : Corporate volunteerism is a form of corporate philanthropy: employees volunteer in nonprofit organizations that are selected by corporations, and they do so under the supervision of the corporation, during working hours or on their free time. We analyze corporate volunteerism, both in France and in the United States, as a corporate policy, focusing on the roots of such policies, on the actors who develop them, on the actors targeted by them, and on their consequences. This thesis is divided into two parts. The first describes the historical, political and social contexts in which corporate volunteerism policies have emerged. The main themes are the importation by some French corporate officers of US volunteerism policies, American philanthropic culture and history, French paternalism, French and American nonprofit sectors, and the ways the American and French governments have promoted corporate volunteerism through tax policies and more symbolic measures. The second part focuses on the different actors encountered during fieldwork conducted in the Paris and New York areas: corporate officers in charge of philanthropy and volunteerism, corporate volunteers, union leaders and nonprofit workers in charge of corporate relations and volunteerism. This dissertation emphasizes the way volunteerism has been used by firms to promote their internal and public image, the transformations of the nonprofit sector as a result of corporate volunteerism, the priority given to corporate goals over social needs and the involvement of the corporate world in matters concerning the public good.