This brief provides an overview of the monitoring and evaluation (M&E) system of the CGIAR Research Program on Aquatic Agricultural Systems (AAS) and describes how the M&E system is designed to support the program to achieve its goals. The AAS program aims to improve the lives of 22 million people dependent upon aquatic agricultural systems by 2024 through research in development. The program's approach is built on six key elements: commitment to people and place, participatory action research, gender transformative research, learning and networking, partnerships, and capacity building. The program works with stakeholders in geographically defined hubs to tackle development challenges relating to aquatic agricultural systems. The brief is organized into three sections. First, we present the objectives of the AAS M&E system in keeping with the key program elements. The second section introduces the theory we draw upon to design the M&E system, and in the final section, we describe the system components.