Learning processes lie at the heart of our understanding of how firms build capabilities to generate and sustain competitive advantage: learning by doing, learning by exporting, learning from competitors, users, and alliance partners. In this paper we focus attention on another locus of learning that has received less attention from academics despite popular interest: learning by supplying. Using a detailed panel dataset on supply relationships in the mobile telecommunications industry, we address the following questions: What factors contribute to a firm's ability to learn by supplying and build technological and market capabilities? Does it matter to whom the firm supplies? Is involvement in product design important, or is manufacturing the key locus of learning? How does a supplier's initial resource endowment play into the dynamic? Our empirical analysis yields interesting findings that have implications for theory and practice, and that suggest new directions for future research.