A number of claims have been made by the developers of program design methodologies, including the claim that the code produced by following the methodologies will be more understandable and more easily maintained than code produced in other ways. However, there has been little empirical research to test these claims. In this study, student and professional programmers were asked to make either simple or complex modifications to programs that had been generated using each of three different program structures. Data on the programmers' modification performance, cognitive representations formed of the programs and subjective reactions to the programs suggested that problem structure (as created by the different methodologies), problem content, complexity of modification, and programmer experience all play a crucial role in determining performance and the representation formed.