Scientific study of how and why humans change over the course of their life. Help parenting, improve child well-being, understand human nature. Study during industrial revolution: child labour socialists concern for child well-being. Plato: children born with innate knowledge, sensory experiences trigger knowledge they have had since birth, no learning necessary. Aristotle: denied innate knowledge, knowledge rooted in sensory experience, children acquire knowledge piece by piece based on info provided by senses. Infant is tabula rasa (blank slate: experiences mold into unique individual, discipline before freedom, similar to locke. Rosseau: children born with innate sense of justice/morals unfolds naturally, max freedom to explore interests, let child develop naturally, similar to plato. Individuals differ: some individuals better adapted to particular enviro more likely to survive pass genes. Baby biographies: detailed systematic observation of children, subjective. Approach uses developmental research to promote healthy (particularly vulnerable families: ensure policy issues/options based on research, organizations (society for research in child development.