While development occurs in a similar fashion for all chil- dren, developmental differences are the inevitable result of indi- vidual genetic and experiential variations and differing cultural and social contexts. In the past several decades, variability has been taken more seriously by social scientists who study children. From that research base we are learning ever more about the mag- nitude and sources of variation among children. Chapter 2 suggested that development is fostered when a child is engaged in activities (both cognitive and social) that are at an appropriate level of difficulty: challenging, but within the reach of the child’s competence. We suggested further that devel- opment is very much dependent on context, and that an adult who is responsive to the child’s level of social, emotional, and cognitive development is a key feature of a supportive context. The research reviewed in this chapter suggests the variability of competencies in children by the end of the preschool years. In both cognitive and social skills, and in the physical and motor development that support those skills, young children vary enor- mously. Biology’s contribution to temperament, learning style, and motor facility clearly influences children’s developmental path- ways. To effectively foster growth in children with very different temperaments, learning styles, activity levels, and abilities to at- tend will require different types of interaction and opportunities to learn.