There is increasing acknowledgement that teachers' knowledge for teaching mathematics is multifaceted and topic specific. Given the paucity of research on the teaching and learning of financial mathematics in general, little can be known about teachers' knowledge for teaching compound interest. However, since financial mathematics is a component of the school curriculum in South Africa, and an important element of financial literacy more broadly, attention needs to be given to knowledge for teaching financial mathematics, and compound interest in particular. Drawing from a larger study in which the author taught a financial mathematics course to pre-service secondary mathematics teachers, a theoretical elaboration is provided of the underlying mathematics of compound interest, and connections with the world of banking. Based on findings from the study, two key student errors are identified: the over-generalisation of linear thinking in multiplicative scenarios, and the over-generalisation of reversible operations in percentage-change scenarios. Taken together, teachers' knowledge of relevant mathematics, of the banking context and of learners' conceptions will contribute to building a knowledge-base for teachers' knowledge for teaching compound interest.