The Maipo basin holds 40% of Chile's total population and almost half of the country's Gross Domestic Product. The basin is located in the semiarid central region of the country and, aside from the typical pressures of growth in developing country basins, the Maipo river faces climate change impacts associated with a reduction in total runoff and changes in its seasonality. Surface water is the main water source for human settlements and economic activities including agriculture. In 2012 a group of researchers from the Universidad Catolica in Chile and other Universities in Chile and abroad, with funding from the International Development Research Center, started a research project to create a climate variability and climate change adaptation plan for the basin. The pillars of the plan are co-produced by researchers and a Scenario Building Team (SBT), which include relevant water and land use stakeholders in the basin. Following similar experiences in other regions in the world that have faced the challenges of dealing with long term planning under uncertainty, the project has divided the task of developing the plan into a series of interconnected elements. A critical first component is to work on the desired vision(s) of the basin for the future. In this regards, the "water security" concept presents the framework that accommodates all objectives of the SBT members. Understanding and quantifying the uncertainties that could affect the future water security of the basin is another critical aspect of the plan. Near and long-term climate scenarios are one dimension of these uncertainties that are combined with base development uncertainties such as urban growth scenarios. A third component constructs the models/tools that allows the assessment of impacts on water security that could arise under these scenarios. The final critical component relates to the development of the adaptation measures that could avoid the negative impacts and/or capture the potential opportunities. After two years in the development of MAPA (Maipo: Plan de Adaptacion) a series of results has been achieved in all critical components that are presented here with special focus on the urban and peri-urban conditions in the basin. The success in the process now poses a series of new challenges, most importantly: how to institutionalize a multi stakeholder process able to implement and monitor the evolution of the adaptation process.