Over the last decade, the field of early intervention/early childhood special education (EI/ECSE) has emerged as a primary service for infants and preschool children with disabilities and their families. Systems for providing early intervention for infants and toddlers exist in every state, and all state Departments of Education are responsible for special education for preschool children. In EI/ECSE, a unified theory of practice has emerged and draws from a range of psychological and educational theories. A strong, evidence-based set of practices that service providers and caregivers use to promote the development and well-being of infants and young children with disabilities and their families underlies this theory of practice. The purpose of this article is to describe the tenets of this theory and identify evidence-based practices associated with each.