This chapter contains a brief introduction to the concept of development. Development is not just a function of nature or nurture, but the interaction between biology and the environment. The most powerful interactions between prenatal and age 3 are the relationships that infants have with parents, caregivers, and other adults in their environment, as well as health supports and risks, such as nutrition, water, sanitation, and disease, along with culture and economy. One model of development in particular that is highlighted is the ecological model, which comes from Urie Bronfenbrenner, and puts the child at the center surrounded by the many influences that affect development – the biosystem, microsystem, mesosystem, and macrosystem. The chapter notes that in discussions of development, there are different domains: the physical, cognitive, language, social, and emotional.