Since the end of the Cold war, Swedish security policy has turned towards building security in cooperation with others. The so-called declaration of solidarity, adopted in 2009, stipulates that Sweden will not remain passive if an EU member state or a Nordic country is attacked, and expects these countries to act in the same manner should Sweden be attacked. Thus, the capability to provide and receive military support - which this report names 'third-party cooperation' - has become a central element of Swedish security policy. This report seeks to deepen the understanding of third-party cooperation, and possibilities and shortcomings related to third-party cooperation towards 2045. In short, it consists of two parts. Firstly, it provides an overview of how third-party cooperation has become a vital part of Swedish defence; how to obtain efficiency; and how Norway plans for third-party cooperation. Secondly, the report lays a foundation for further studies by developing and utilising a methodology for assessing future possibilities and shortcomings related to third-party cooperation in defence. The analysis focuses on military support to Sweden or to others with Sweden hosting the support effort. The report is a first step towards future research on the subject, which may include more specific themes, and support to war games or scenario-based discussions related to third-party cooperation.