The processes of curricula, textbooks and student resources development have been broadly surveyed and studied while teachers' guides have received comparatively little consideration throughout recent reforms in science education. Ideal curriculum materials align instruction with the goals of reform. Well-designed teachers' guides contribute to communicating and supporting reform-based teaching. The purpose of this study was to investigate the functions and assistance that guides provide science teachers. A questionnaire was developed and administered to science teachers from a stratified sample of elementary and junior high schools in Taiwan. The findings indicated that the guides were of greater benefit to elementary school science teachers than they were to junior high teachers. These groups of teachers perceived the guides' usefulness differently, but they believed that a clear, concise presentation of the purpose, reminders, answer keys, concept maps of the main topics and learning progression provided the most helpful and preferred layout. Teachers felt that the function of a teachers' guide is to provide teaching resources rather than to guide teacher thinking. This study discusses design suggestions that can engage teacher thinking.