Due to the emergent and dynamic nature of incidents, the complexity of emergency work is often referred to as a challenge for learning. Another recurrent challenge in emergency work is that of collaboration at and across established organizational boundaries involving actors with specific types of expertise who are operating under different regulations and responsibilities. In addition, training emergency service organizations in collaboration remains a challenge. In light of the difficulties and shortcomings that have been identified in major incident responses, the need for exercises for developing and maintaining collaborative response effectiveness prior to the next incident is often highlighted. The overall aim of this thesis is to understand how full-scale exercises can provide conditions for developing inter-organizational collaboration between the police, ambulance and rescue services at the incident site. Learning activities that carry the potential to support and develop collaborative capacity, and how the alignment of distributed expertise can be trained for, were of particular interest. Interviews with participants in eight full-scale exercises with professionals and interviews and observations of one exercise with senior-level students in Sweden served as the empirical base.Central concepts from Cultural-Historical Activity Theory (CHAT) provided theoretical tools to explore the exercises and understand boundaries between organizations with a wider, systemic approach. The empirical studies show that the participants understood full-scale exercises to be valuable opportunities for becoming involved in response work, making decisions, and acting and interacting in uncertain situations and realistic environments. As in real-life responses, exercises are characterized by the stabilization and confirmation of everyday routines on the one hand, and by improvisation and change on the other hand. The studies also show that exercises tend to focus on specific scenarios,intra-organizational routines, and leadership positions. Infrequent exercises inwhich the participants were only trained in a limited role were perceived to be in adequate for developing preparedness and collaboration. However, the analysis suggested that the way in which exercises were organized and performed had implications for how participants were trained in collaboration.Realizing the potential of boundaries as resources for learning in exercises depends on how boundaries are explicated and approached. Thus, rather than striving to ignore or eliminate boundaries in exercises, the studies illustrated the learning value of explicitly reflecting on the multiple understandings around boundaries. The studies demonstrated that much of the work at an incident site takes place around negotiations. Collaboration at the incident site was not only aquestion about boundary crossing; operational tasks may not always be aligned and have to be prioritized and sequenced. The exercises comprised work situations in which no single motive could explain or determine the collaboration,due to different types of expertise, primary responsibilities and needs forinformation. These factors were understood in terms of the concepts of boundarywork and boundary awareness. These concepts point at a more divergent understanding of collaboration that reaches beyond striving to create mutual understanding between organizations in learning activities. Differences between organizations, such as in terminology, time horizons, priorities, leadership structures, understandings of safety and how intra-organizational decisions and actions could impact the collaborating organizations' work, were central triggers for discussion and negotiation. These differences required explanations in order to make the actions and decisions of one organization understandable and justifiable to another, based on organizational mandates and types of expertise.Giving emergency services the opportunity to work together, to develop an awareness of their expectations of each other in various situations, to use and interpret their own and others' terminologies, and to identify internal hierarchies and motives for prioritizations was essential dimensions of exercises