Studies of over 100 chess players at varying skill levels and ages show the ways in which experts and nonexperts differ in problem-solving strategies. Important differences are found at all stages of problem solving. The most significant differences appear to be before and after the evaluation of alternatives ("sizing up" the problem, generating alternatives, and evaluating outcomes). Insight into how expertise develops and how it can be fostered can be gained by following the development of young players with high potential. Discussion focuses on expertise, stages of problem solving, identification of experts and potential experts, and training. (RH)