This chapter focuses on some of the theorists who have shaped the way practitioners think about early years and, more specifically, their work with those children who fall into the category of toddlers and pre-school children. The most important element among the theory is the child. Children are born curious, eager to learn about the world and wanting of social connection. Children learn in a variety of ways; through the right environment and interaction with adults, they will make progress. John Dewey’s greatest contribution to modern early years practice is the now widely accepted notion that children learn through experiences and through their interests. Influenced by Dewey, Susan Isaacs was a psychologist whose theory of child development focused on the importance of play and how children learn through social interaction with others. Erik Erikson was a developmental psychologist best known for his psychosocial theory, describing different stages that humans go through to develop within a social world.