One of the great nineteenth-century psychologists, William James, compared consciousness to a river, and coined the famous phrase 'stream of consciousness'. This chapter looks at different theories and aspects of cognitive development in a way that is scientifically rigorous, but also accessible to those who love, live and work with children – psychologists, teachers, doctors, nurses, social workers, students and parents. For more than 125 years, psychologists have tried to understand the stages by which children's thinking develops; some have also tried to understand the fundamental causes of development and have argued over whether our intelligence is a matter of nature or nurture. The classic texts in child psychology – especially those of the great Swiss psychologist Jean Piaget – come from a time when education was formal. The chapter provides exercise that may help convince sceptics who think they don't think or those who don't think they think. It also presents an overview of the key concepts discussed in this book.