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City governments are at the forefront of public problem-solving. As they deliver services to residents and work to tackle complex social challenges, they often coordinate and communicate with each other. Against the backdrop of globalisation and urbanisation, which have been documented and analysed widely, the phenomenon of inter-city collaboration (ICC) has remained understudied. In this article, we examine existing and emergent patterns of how and why cities collaborate. We conducted a desk review of ICCs in 64?US cities. We interviewed city leaders in US and international cities (N?=?13) and leaders of ICCs (N?=?20). We also surveyed leaders from 47?US cities. Through our analysis, we put forth an analytical framework of ICCs, describing the forms they take and the value they deliver. We identify various forms of value ICCs produce for cities and examine under what conditions they are likely to be effective. While not all collaborations are successful, ICCs have the potential to generate substantial benefits through mutual learning, economies of scale and the power of collective action. Intentional design and management of ICCs can make them effective platforms of ‘horizontal governance’ and solving collective action problems. Policy implications Inter-city collaboration (ICC) is increasingly necessary for cities to address new wide-reaching challenges arising from rapid urbanisation, social and political change and unclear ownership over shared problems. Distinguishing the form ICCs take (e.g. issue-based coalitions, operational partnerships) and the value they deliver (e.g. joint policymaking, regional coordination) can help city leaders decide which ICCs to enter and exit in order to advance policy goals. ICCs play an important role in reducing barriers for city leaders to experiment and adopt innovative policy solutions. Through participation in ICCs, city leaders can develop and amplify a unified voice in state, national and global policy.