The ability to assess and demonstrate organizational performance (the ability of an organization to use its resources efficiently and to produce outputs that are consistent with its objectives and relevant for its users) is a key issue in many agricultural research organizations. This Research Management Guideline introduces to an audience of research managers and research institute staff the main concepts underlying performance evaluation of organizations, and the methods and procedures for implementing the Organizational Performance Assessment System (OPAS) for agricultural research organizations. OPAS has been developed and refined over the years by the ISNAR in close collaboration with national agricultural research partners in developing countries. A rapid and cost-effective method, OPAS represents a shift in perspective from an externally driven evaluation of programs or projects, often done for the benefit of outsiders, to an internally driven assessment, aimed at improving performance and accountability. The OPAS evaluates performance at two levels: it reviews the productivity of organizational outputs, and it addresses crucial internal management issues that affect performance. Output assessment and management assessment are the focus of this publication.