This study explores the nature of professional expertise in economics. Two experiments were conducted to examine the existence of problem categories in economics as a basis for problem representation. Findings are that experts use categories that reflect major economics principles, and novices sort by literal cues contained in the problems. In the second trial, experts did not produce stable sortings whereas novices produced a stable sorting of categories. Subjects included 6 experts, 6 intermediate fourth-year university students in economics, and 6 novices in the first year of college with one semester in economics. Subjects were asked to sort 18 economics problems into groups based on similarities of solution. After sorting the problems, subjects were asked to write down the problem-numbers and give a description of the category. A second study was to determine the stability of sorting procedures among experts after a lapse of six weeks. No differences were found in the average number of categories produced per sort between different groups. (EH)