Numerous Chemistry Education Research studies have characterized students' understanding of organic chemistry mechanisms as disparate collections of memorized patterns of surface features rather than useful explanatory and predictive models of structure–property relationships. This chapter synthesizes our research leveraging causal mechanistic reasoning to engage students in scientific modelling about reaction mechanisms. We unpack our definition of causal mechanistic reasoning, discuss evidence-based strategies to engage students in causal mechanistic reasoning and modelling, and review findings from two longitudinal studies measuring students' engagement in these practices overtime in different curriculum contexts. We conclude by identifying pedagogical strategies to integrate causal mechanistic reasoning and modelling practices into chemistry courses.