Formative assessment is designed to provide information about students' learning to help them and their teachers to identify deficiencies and misconceptions. It differs from summative assessment, which aims to rank students according to their achievements to determine which students pass or fail or to assign grades to students. This article reports on a symposium concerned with evidence for the effectiveness of formative assessment in improving learning. It was presented by the Teaching of Physiology Section of the American Physiological Society at the Experimental Biology Meeting of 2008.