The main aim of this paper is to report on an investigation into primary pre-service and inservice teachers' content knowledge of decimals. The participants were asked to complete four decimal tasks including the ordering of decimals, operating with decimals and converting a fraction to a decimal. The results from both sets of participants are compared and incorrect answers are described and discussed. The findings indicated a reliance on formal procedures and that incorrect responses indicated a fundamental lack of understanding of place value and the presence of misconceptions that are commonly held by their student counterparts. The study contributes to the limited field of research that uses comparative studies to look specifically at pre-service and in-service teachers' knowledge of decimals. (Contains 2 figures, 7 tables, and 1 footnote.)