The articles in this MERJ special issue collectively explore how spatial reasoning is related to, and supports, mathematics learning. Spatial reasoning refers to a suite of different skills that involve the mental manipulation of two-dimensional and three-dimensional relations between and within objects. The relation between spatial reasoning and mathematics achievement across development is consistent, predictive, and strengthens over time (Resnick et al. 2019). Fortunately, spatial reasoning can be learned and improved with practice (Lowrie et al. 2018; Uttal et al. 2013), and improvements in spatial reasoning are causally related to improvements in mathematics understanding (Cheng & Mix 2014; Lowrie et al. 2017). However, these relations vary by task (e.g., Mix et al. 2016), highlighting the need to consider how specific connections are formed and extend to more general performance.