Marine Spatial Planning (MSP) has evolved over many years and since its early beginnings there has been a growing urgency to develop transboundary planning. This is because the borders of marine ecosystems and the dynamics of some maritime activities, such as navigation, are not restricted to or bound by specific political and administrative borders. Cooperation across borders has been promoted by higher political levels for decades, and the implementation of cross-border consultation procedures is regulated by law. However, literature suggests that transboundary interaction is not an obvious step in the process of MSP and that today's practices have various weaknesses. This paper examines current practices and procedures of transboundary MSP interactions in the Baltic Sea Region to date. It brings together results from MSP process observations and interviews with marine planners in two recent research projects (Baltic SCOPE and BONUS BALTSPACE). Our results confirm the need for transboundary interaction and integration. The research also shows that there are differences in how MSP agencies interact with domestic and foreign stakeholders. Furthermore, formal transboundary consultations often seem to be limited to topics of the environment and health, and to the stakeholders responsible in these realms. The results include a variety of ways to overcome these challenges.