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 threedimensional 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. This special issue brings together international research across the fields of mathematics education, development, and cognition, which serves to bridge and bring exposure to frequently siloed findings. In doing so, we (the editorial team) were faced with many challenges reconciling the fundamentally different methodologies, definitions, and valued outcomes each field takes in their research practice. For example, we needed to balance finding appropriate reviewers to ensure the accepted papers would meet the methodological standards within and across the relevant fields of discipline, while purposefully showcasing representative approaches that might complement or be adopted by researchers from other fields. To this end, the articles within the special issue have taken care to explicitly describe their techniques and define their constructs. We have emerged with a collection of nine strong papers across four continents. Contributions range from early career to experienced researchers; such is the breadth and depth of research on this topic. These papers can be conceptualized along two overarching themes: (1) theoretical/ conceptual links between spatial reasoning and mathematics skills and (2) practical application of spatial reasoning learning in mathematics environments. The Lowrie