Abstract:This study examines secondary school students’ understandings and misconceptions when working with logarithms using a specially designed test instrument administered to 81 students in two Singapore schools. Questions were classified by cognitive level. The data were analysed to uncover the kinds of errors made and their possible causes. Students appear capable of doing routine calculations but less capable when answering questions which require higher levels of cognitive thinking. In addition, many errors are not due to lack of knowledge but appear to be based on over-generalisation of algebraic rules. Suggestions for practice based on these findings are provided.