Students' construction, coordination, and abstraction of units underlie success across multiple mathematics domains. Structures for coordinating units underscore notions of numbers as composite units (e.g., five is a unit of five and five units of one). In this working group, we seek to facilitate collaboration amongst researchers and educators concerned with units construction and units coordination. The aim for this working group is two-fold: (1) to extend our research around units construction and coordination to new grade levels; and (2) to collaborate with researchers who investigate students with learning differences in school settings to determine diverse students' mathematics learning trajectories. [For the complete proceedings, see ED606556.]