Young students, new to formal mathematics, come to school with various number experiences. To meet their needs, teachers must understand each student's thinking and tailor instruction appropriately. Differentiation is a vehicle for making this possible. According to Tomlinson (2001), differentiated instruction is student-centered; rooted in ongoing assessment; provides multiple access points; and allows for several approaches to problem solving. Unlike ability grouping, differentiation encourages teachers to maintain high expectations for "all" students while recognizing individual needs. Lesson study is one structure that supports teachers' efforts to improve their lessons. Through lesson study, teachers plan a lesson or unit, observe one another teaching it, and continually reflect on students' learning in relation to the lesson supports. Additionally, teachers develop a "researcher lens," which they use to examine their practice, and have the opportunity to focus on and discuss student thinking. In this article, the authors describe the process through which three first-year kindergarten teachers developed a unit on combinations of ten, which emphasized high expectations for all students while addressing their varied learning styles. They then focus on the results of one lesson in the lesson-study cycle and highlight how differentiated instruction supported student access and learning and how the process influenced planning and promoted teacher growth. (Contains 2 tables and 3 figures.)