The research pertaining to concept formation is reviewed and reinterpreted. Two methods (an interview-questionnaire technic and a performance situation) have been used in investigation of concept formation in children. With adults, the methodology has included introspection, learning situations, and problem solving. With reference to conceptualizing ability in general, children can and do form concepts at a very early age. The chief differences between children and adults are of degree rather than kind. "Concept scores" have low correlations with intelligence but correlate highly with measures of training and experience. With increasing age, concepts become more numerous, more complex, and more "logical." There is some evidence that children acquire concepts in a definite order. The formation of a particular concept is influenced by (a) different ways of presenting the stimuli (order of presentation, complexity), (b) contextual factors, (c) the inclusion of negative as well as positive instances, (d) set, (e) interference between similar series. Among the characteristics of concept formation noted are the tendency to evolve concepts gradually, the occasional attainment of concepts without awareness of the concept itself, the tendency for concepts to be learned in a definite order, and the occurrence of varying degrees of concrete and abstract approaches to, and use of concepts. Seventy-six studies are reviewed.