In four parts, this discussion describes characteristics of the thought of infants, preschool children, primary school students, and adolescents. Topics briefly addressed in part I, on the thought processes/capabilities of babies, concern sensorimotor thought without abstraction, the importance of physical exploration, the development of intentionality, learning about cause and effect and properties of objects, and the development of symbolic thought. Table 1 describes motor development in the first 3 years, and Table 2 delineates six stages in the development of object permanence. Discussion of the development of preschool children in part II focuses on combining and transferring information; reversing observations; egocentricity; animism and artificialism; intuition and sequential associations; cause and effect; classification; and applications (specifically concerning preschoolers' drawings, dreams, and concepts of death, as well as professional practice, preoperational thought in adults, and the transitional years). Table 3 illustrates conservation of liquid, number, matter, length, area, and volume. The development of the primary school student is described in part III in terms of the extension of conservation, seriation, classification, concepts of number and time, concrete thinking, verbal problems, hypothetical problems, proportions, combinatorial logic, and applications to primary school children's social concepts. Educational implications are also discussed. In part IV, adolescents are discussed with reference to the egocentric idealistic crisis; combinatorial logic; hypothetical deductive thinking; impression formation; use of symbols for symbols; and, very briefly, thought beyond formal thought. (RH)