This report summarizes research about development during the first three years in order to highlight areas that are foundational for later school readiness and success. This information can be used as a guide by programs to inform their practices and policies and to help programs think about their own theories of change or strategies for continuous improvement and what outcomes they are most focused on improving for young children. Until recently the term school readiness has typically been applied to preschool-aged children, but it is now increasingly being used in relation to infants and toddlers. With this shift comes the knowledge that birth to age 3 is a time of unparalleled growth and change that provides special opportunities to support school readiness, and school readiness for this age group must be defined with those particular characteristics in mind. (author abstract)