Information systems are seen as strategic business tools, frequently essential to a firm and central to its competitive strategy. Their importance is now acknowledged. But information technology -- equipment and services -- is available to all firms, and most applications can be duplicated; often the copying firm enjoys the advantages of newer and better technology, learns from the experience of the innovator, and offers comparable services at reduced costs. When can an information system convey sustainable competitive advantage? We believe that the benefits resulting from an innovative application of information technology can be defended if: o they are so closely tied to the strategy of the innovating firm that competitors do not wish to copy them o they exploit unique structural characteristics of the innovating firm -- aspects of vertical integration, degree of diversification, or unique skills and resources -- so that competitors do not benefit from copying them We introduce here a model of the firm, based on value chain analysis, that highlights differences among firms; the model then guides the search for defensible opportunities for competitive advantage that exploits these differences.