Marine Protected Areas (MPAs), particularly no-take marine reserves, can be powerful tools for conservation and management. However, they are only effective if they are well designed and effectively managed. To ensure that MPAs will benefit both people and nature, they need clear objectives and design criteria that address biophysical, socioeconomic, cultural and governance considerations.  I will review the biophysical criteria for designing MPAs so they will enhance fisheries and protect biodiversity in the face to climate change, and describe how we have used this science based approach to help improve the design of MPAs in tropical marine environments in over 20 countries worldwide.