This research aims to investigate the algebraic under performance of second year post-primary students in Ireland (approximate age 14 years). To this end a diagnostic test for algebra has been developed to profile and identify students who are struggling with algebra. This paper examines the development of the test, which involved the identification of key mathematical content areas that are critical for success in algebra. Both prerequisite, and algebra content areas are key to a students’ success in algebra and how each of these areas contribute to a students’ progress with algebra is discussed in this theoretical paper. Test items have been selected and adapted from the literature which are aligned with both the key content areas and the Irish mathematics syllabus at junior cycle, the initial three years of post-primary education in Ireland.