According to Jean Piaget, intelligence develops in a necessary sequence of stages that are related to age. Piagetís theories have given us profound insights into the cognitive development of children. This knowledge in turn, provides a framework for understanding how children think and for planning educational strategies. Using structured interviews with children from four to nine years old, Dr. David Elkind illustrates the development of transitive thinking and reversibility as children move from the pre-operational to concrete operational stage of cognitive development. Childrenís construction of the unit concept, so basic to arithmetic and beginning reading, is illustrated in a well run first grade class. Dr. Elkind also examines some recent criticisms of Piagetís theories.