Several models for problem solving are discussed, and the idea of a heuristic frame is developed. This concept provides a description of the evolution of problem-solving skills in terms of the growth of the number of algorithms available and increased sophistication in their use. The heuristic frame model is applied to two sets of physical problems to illustrate the components involved. Several teaching strategies related to the heuristics and to promoting students' self awareness of their developing problem solving ability are discussed. In an appendix, the problem-solving model is related to the Piagetian idea of conservation. (SD)