The growing trend towards partial or total abandonment of conscription for more or less broad forms of professional volunteerism in armed forces (see the chapter on the “Decline of the Mass Armies” by Karl Haltiner in this regard) is giving renewed importance and topicality to the study of union representation of the interests of military personnel. The subject attracted widespread interest among military sociologists in the 1970s before being substantially abandoned. It thus seems opportune to give some space in this handbook to the social and political issues that the unionization of professional military personnel can involve and to report the most significant studies published on the subject.