We report the development of an assessment instrument that provides a profile of the attainment and the errors of pre-service primary teachers across the mathematics curriculum. We describe test development, analyses and test validation involving a sample of 426 pre-service teachers in the first year of their training in primary education courses in Australia. We discuss a range of errors, strategies and misconceptions made across different strands of the mathematics curriculum and show that pre-service teachers are making the same errors as children. A second sample of 86 pre-service teachers in England was used to validate the test. We also describe how these pre-service teachers in the second year of their program made sense of their personalised diagnostic profile from the test in order to develop their mathematics subject knowledge. (Contains 9 tables and 5 figures.)