For more than a century, researchers have surveyed the specific mathematics skills of children entering school. With increasing numbers of children entering preschool (especially programs designed for children at risk), there is a need for such studies of younger children, especially those from low-resource communities (LRC). We review previous work and report two studies investigating the mathematics knowledge and competencies of children entering preschools in two states in the U.S., using theoretically-based assessments emphasizing psychological developmental progressions. Results suggest that children are acquiring mathematical concepts and skills at younger ages than previous generations. Children from LRC enter preschool with a range of mathematical skills and concepts upon which educators can build, but are not achieving their full potential. These results are intended to assist those responsible for developing standards, writing curricula, and assessing and teaching all children by providing updated information about what children know when they enter school, including the specific levels of achievement along research-based developmental progressions.