Rational number knowledge is critical for mathematical literacy and academic success. However, despite considerable research efforts, rational numbers present perennial difficulties for a large number of learners. These difficulties have led some to posit that rational numbers are not a natural fit for human cognition. In this chapter, we challenge this assumption, describing recent research into intuitive routes to understanding rational number concepts that diverge from those popular in current curricular recommendations. Namely, we develop the claim that humans are perceptually sensitive to nonsymbolic ratio magnitudes, and that this sensitivity is an early developing, robust and abstract aspect of cognition. We suggest that attending to this perceptually based sensitivity can inform existing theory and help provide a basis for the design of more effective instruction on rational number concepts. [For the complete volume, "Constructing Number: Merging Perspectives from Psychology and Mathematics Education. Research in Mathematics Education," see ED616587.]