The propensity to use a spatial framework to organize other pieces of information is a widespread phenomenon that permeates humans' representation of diverse concepts, including numerical quantities. Developmental studies on numerical cognition have revealed that humans possess a system for abstract quantity representation that is functional at birth and connects to a spatial representation system. Human infants, children, and adults link increases and decreases in numerical quantity to corresponding increases and decreases of spatial extent, as well as to lateralized right/left spatial positions, respectively. In this article, I discuss the origins of number-space mappings, their presence throughout development, and their functional properties. I also argue that number-space mappings reflect inborn biases, possibly shared across other species, that support both efficient magnitude processing and serial learning.