A growing body of evidence reveals the need for research on, and consideration for, children's and students' own--self-guided--spontaneous use of mathematical reasoning and knowledge in action. Spontaneous focusing on numerosity (SFON) and quantitative relations (SFOR) have been implicated as key components of mathematical development. In this chapter, we review existing research on SFON and SFOR tendencies in the broader context of the development of mathematical skills and knowledge and examine how the state-of-the-art evidence on SFON and SFOR is relevant for the field of mathematics education. We discuss individual differences in SFON and SFOR, associations between spontaneous focus on mathematical features and mathematics achievement, the contributions of situational contexts that implicitly prompt attention to number, and ways to increase children's focus on number regardless of their baseline level tendencies. We conclude that children's and students' tendencies to focus on number and quantitative relations--spontaneous or otherwise--are key components of mathematical development and education. [For the complete volume, "Constructing Number: Merging Perspectives from Psychology and Mathematics Education. Research in Mathematics Education," see ED616587.]