This paper presents and discusses legal, methodological and political frameworks for the development of the proposed Portuguese Marine Spatial Plan initiated in 2008. It considers lessons learned and is informed by discussions that have taken place since publication of the ‘Roadmap for Maritime Spatial Planning: Achieving Common Principles in the EU’. New goals are based on horizontal planning tools that cut across sea-related sectoral policies and support joined up policy making. It is in this context that Marine Spatial Planning (MSP) emerged as an essential process for sustainable decision making. The OSPAR Commission undertook an overview of national planning systems within its administrative boundaries, which confirmed spatial plans reduced conflicts. However, problems exist accessing good quality data and dealing with entrenched sectoral views. Furthermore, the transboundary nature of marine resources requires cooperation between neighbouring states. In 2006, Portugal developed a ‘National Sea Strategy’ that recognized the importance of developing its maritime space while valuing marine habitats and biodiversity. MSP development of the Portuguese sea commenced in 2008 and findings are now evaluated. They showed adaptation of existing tools to be possible and desirable, provided undertaken cautiously and found conceptual ambiguities were barriers to conflict resolution. Furthermore they showed management strategies should be designed and analysed on a case by case basis, recognising temporal and spatial variations.