Kindergarten mathematics skills are important for subsequent achievement, yet mathematics is underemphasized in kindergarten classrooms. Using nationally representative data, this study explored the relationship between students' school-entry math skills, classroom content coverage, and end-of-kindergarten math achievement. Although the vast majority of children entered kindergarten having mastered basic counting and able to recognize simple geometric shapes, their teachers reported spending the most mathematics time--typically about 13 days per month--on this content. On average, exposure to this basic mathematics content was negatively associated with math achievement across kindergarten. Importantly, children with the lowest levels of math skills benefited from exposure to this basic mathematics content whereas other children benefited from exposure to more advanced content. (Contains 7 tables, 1 figure, and 3 notes.)