As part of the reform process of the Consultative Group on International Agricultural Research (CGIAR), its Technical Advisory Committee is being transformed into a Science Council. An interim Science Council has been operating since January 2002. The primary role of the Council will be to enhance and guard the relevance and quality of science in the CGIAR System. Relevance is viewed as researching the "right things" to address the System's goals, and deals with the relevance of identified research priorities and strategies and the nature of the planning and consultation process. Ex-ante analyses of need and potential impact to formulate the priorities and strategies are important elements in defining relevance. Quality is viewed as researching the "right things well" and deals with the efficient use of state-of-the-art knowledge, research methods and protocols. The paper outlines the interim Council's suggestions for an operational framework of the Science Council. It serves as a starting point for the development of specific criteria, procedures and guidelines for carrying out the envisioned responsibilities. The Science Council must act in a strategic advisory role, basing its advice on: (i) planning and strategy development in the context of CGIAR goals; (ii) internal self-assessments by CGIAR Centres and Programmes and independent external monitoring and evaluation; and (iii) impact assessments. The paper discusses these three complementary functions. Each depends on the other in a dynamic feedback process that is essential if the CGIAR is to remain at the frontiers of science and development, maintain its science relevance and quality, and contribute in the most effective and efficient manner to the mission and goal of the Group.