For pre-service teachers to develop learners' number sense, they need have a broad repertoire of non-standard calculation strategies. In this paper, we present an analysis of selected research literature from 2000 to 2022 on pre-service teachers' mental mathematics published in peer-reviewed journals. The results of this brief review indicate that more research on pre-service teachers' mental mathematics would be beneficial. Of the 12 studies examined, five promoted a deficit view of pre-service teachers' knowledge. The remaining seven differed on the starting point in developing pre-service teachers' knowledge of non-standard mental calculation strategies.