This topic area develops a common framework, building on existing research, for the assessment of aquatic ecosystems that is needed for the development of management concepts. It first frames the AQUACROSS Assessment Framework based on relevant conceptual frameworks (i.e. MAES). Building upon this, the actual AQUACROSS Assessment Framework is established, focusing on concepts, tools and methods and following the DPS-EGS-IR-resilience causal framework, as well as integrating crosscutting issues such as resilience thinking, uncertainty, dealing with varying spatial and temporal scales, and data and metrics for indicators. The Assessment Framework is further refined and updated based on feedback from other topic areas and implementation in the case studies.