In his 1973 article "The Structure of ill structured problems", Herbert Simon proposed that solving ill-structured problems could be modeled within the same information-processing framework developed for solving well-structured problems. This claim is reexamined within the context of over 40 years of subsequent research and theoretical development. Well-structured (puzzle) problems can be represented by a problem space consisting of well-defined initial and goal states that are connected by legal moves. In contrast, the initial, goal, and intermediate states of ill-structured (design) problems are incompletely specified. This article analyzes the similarities and differences among puzzles, insight puzzles, classroom problems, and design problems within Gick's ("Educational Psychologist," 21, 99-120, 1986) theoretical framework consisting of representation construction, schema activation, and heuristic search. The analysis supports Simon's ("Artificial Intelligence," 4, 181-201, 1973) claim that information-processing principles apply to all problems but apply differently as problems become more ill structured.