The purpose of this study was to report the results of a nationwide survey of Taiwanese high schools students' understandings about electric circuits. The study involved two stratified random samples consisting of 7,145 students in Grades 8 and 9, and 2,857 students in Grade 11, accounting for about 2.3% of the total enrolment in the corresponding grades in Taiwan. Participants' responses to the two-tier diagnostic instrument were analysed to identify the possible alternative conceptions that were held by the students. The extent of these alternative conceptions was compared across grades, types of circuits, and propositional statements. Finally, the results were contrasted with the research literature and some pedagogical implications were provided. (Contains 4 tables and 4 figures.)