This introduction presents an overview of the key concepts discussed in the book The Cambridge Handbook of Environment in Human Development. The book explicates the role of environment in children's development. In order to understand the role of environment in the development of the child one need to articulate the various models used in understanding the child by environment interaction. Such models vary from trait views in which the child may impact the response of his or her caring environment to epigenetic models where environments interact at the genome level, producing differences in replication of the genetic code. When environment has been assessed and considered as a variable or condition that not only changes with the child but also impacts the child's developmental trajectory, it is most often defined as the most proximal environment, that of maternal care, especially in the first months and years of life.