This is the final report on a three year project designed to investigate the development of problem-solving strategies in elementary school children. In this project the personality of the child as it is reflected in problem solving style is examined. The hybothesis that reflective children are more competent problem solvers than impulsive children was tested. Some evidence suggests that reflection/impulsivity is related to task-oriented and social behavior in the classroom as well as to individual differences in academic achievement. Four separate tasks were selected which permitted a detailed analysis of hypothesis-testing strategies in sequential problem solving. Each task was administered to a sample of reflective and impulsive 7, 9, and 11 year olds in 1974, and available subjects were retested in 1975 and 1976. The primary objective of this research was to describe the development of problem solving strategies in reflective and impulsive children during the elementary school period. A second major objective for this research was to determine the effects of training impulsive children to use more efficient problem-solving strategies. The final objective for these studies was to assess the construct validity of the reflection/impulsivity dimension by determining whether reflective and impulsive children differ in observed classroom behavior. Collectively, these studies provide rather strong support for the notion that cognitive style, as indicated by response tempo in situations of response uncertainty, reflects individual differences in the development of essential problem-solving skills. (JD)