This document is the first Strategic Research and Innovation Agenda (SRIA) of the Joint Programming Initiative “Water Challenges for a Changing World” (the Water JPI). This is a Public-Public Partnership (P2P) initiative responding to the grand challenge of “Achieving Sustainable Water Systems for a Sustainable Economy in Europe and Abroad”. This SRIA version 0.5 has been obtained through discussions dating back to the onset of the Initiative. Efforts intensified in 2011, with the first consultation of the Water JPI Advisory Boards. In the period of 2012 and 2013, the SRIAs of some JPIs were released, and relevant developments were made in Horizon 2020 and in other new neighbouring initiatives, such as the European Innovation Partnership on Water (EIP on Water). The second consultation of the Advisory boards, early in 2013, marked the beginning of the drafting of this document. SRIA 0.5 is intended to guide the development of Water JPI activities in 2013 and 2014, to inform stakeholders of the status of agenda developments and to contribute to sustaining an effective, mutually beneficial dialogue with Horizon 2020. The release of SRIA 1.0, scheduled for June 2014, will require additional context analysis, information processing, critical review, wide consultation and redrafting. The priority themes identified in this document include: i) Maintaining Ecosystem Sustainability; ii) Developing safe water systems for the citizens; iii) Promoting competitiveness in the water industry; iv) Implementing a water-wise bio-based economy; and v) Closing the water cycle gap. Research, development and innovation needs have been developed for each theme. Envisaged interactions have been described, paying particular attention to areas of overlap with Horizon 2020 where efforts can be pooled, and to interactions with other JPIs and neighbouring initiatives, where joint activities could be planned. The document is completed with discussions on SRIA evaluation and monitoring, and on instruments and plans for agenda implementation.