Collaboration models and tools aim at improving the efficiency and effectiveness of human interactions. Although social relations among collaborators have been identified as having a strong influence on collaboration, they are still insufficiently taken into account in current collaboration models and tools. In this paper, the concept of service protocols is proposed as a model for human interactions supporting social requirements, i.e., sets of constraints on the relations among interacting humans. Service protocols have been proposed as an answer to the need for models for human interactions in which not only the potential sequences of activities are specified-as in process models-but also the constraints on the relations among collaborators. Service protocols are based on two main ideas: first, service protocols are rooted in the service-oriented architecture (SOA): each service protocol contains a service-oriented summary which provides a representation of the activities of an associated process model in SOA terms. Second, a class-based graph-referred to as a service network schema-restricts the set of potential service elements that may participate in the service protocol by defining constraints on nodes and constraints on arcs, i.e., social requirements. Another major contribution to the modelling of human interactions is a unified approach organized around the concept of service, understood in a broad sense with services being not only Web services, but also provided by humans.