A rationale for determining what evaluations of AIDS education programs should assess is outlined. The rationale is based in: educational theory with a distinction made between knowing and understanding and an emphasis on helping learners understand the concepts being taught; and teacher development literature and theories, with assessment of understanding being a crucial element over and above measures of fact recall knowledge. Evaluations should explore what learners understand about AIDS rather than merely what they know. Knowledge and constructivism, issues in assessing understanding, and several AIDS education evaluations in the literature are considered. Data from an evaluation of what students understand about HIV and AIDS are provided. The evaluation focused on HIV and AIDS education in postprimary schools in Victoria (Australia). Surveys of schools were followed by interviews with teachers in one-third of the schools that offered AIDS education. Two questionnaires were administered to 579 students; one assessed knowledge and attitudes and the other explored knowledge and understanding. Selected students were interviewed individually and in a small group. Individual interviews gathered data on 162 students' ability to understand issues conceptually similar to HIV and AIDS as well as on their constructions about health, sexual health, and sexually transmitted diseases. Small group interviews were audiotaped and 27 groups of 6 students each were required to apply their knowledge to a risk assessment task and a poster selection task. Teachers were then reinterviewed. Six data tables, 1 graph, and 78 references are included. (RLC)