In the United States, for typically developing children, age has historically been the most common factor determining when a child starts formal schooling. Recently, there has been increased emphasis on other indicators of being ready for school. Beginning with Head Start in 1965 and mushrooming into state-funded prekindergarten programs in most U.S. states, preschool has been viewed as an intervention to facilitate school readiness for children from low-income families. In 1986 in the United States, an amendment to Public Law 94-142 extended preschool intervention services to children with disabilities. Confusion still abounds, however, about what the concept of school readiness means and how it would translate into classroom practices and school policies that would help children be more school ready. This paper will focus on alternative definitions of school readiness, the measurement of readiness skills, and implications for new practices and policies.