Marine Spatial Planning (MSP) has been heralded as the key means of achieving a more integrated approach to marine use across sectors and spatial scales. Achieving greater integration and coherence in MSP governance arrangements is seen as a way to resolve current problems of marine governance (such as fragmentation) and address future resource demands in a sustainable way. However, there is a lack of clarity and consensus in practice regarding sustainability in MSP, both in terms of MSP governance practices and sustainable resource use. For example, how are we to treat the environment in MSP? Should we conceive the environment as just another sector with interests to be negotiated, or as the very boundary condition that limits possibilities for maritime activities and developments? How do we integrate diverse views on this in MSP decision-making? This is but one example of an integration challenge in MSP important for sustainability. There are numerous others. Integration is intimately connected to the ability of MSP to deliver sustainable marine resource use at various levels and scales. The roles of integration are diverse and interconnected, including those that affect social-ecological integration or land-sea interaction, but also aspects of good governance and social sustainability. The latter include inter-sectoral decisionmaking, stakeholder engagement, cross-border interaction and knowledge pluralism. How integration is exercised in these procedural aspects of MSP is likely to substantively affect outcomes both in terms of sustainable blue growth or the ability to deliver an ecosystem-based approach. Integration as a policy and analytical problem to be addressed has also been discussed elsewhere – most saliently in the fields of sustainable development, ICZM, environmental policy integration, planning theory and socio-ecological systems. While there has been some work on integration in MSP, additional insight is needed: to better empirically ground the roles of integration in MSP, to understand the multidimensionality and interdependencies of integration dimensions and to unpack what ‘balance’ might mean for understanding and pursuing sustainability in different MSP contexts. In response, this special issue aims to explore the roles, problems and opportunities of various types of integration in relation to MSP's sustainability ambitions.