Marketing managers can increase shareholder value by structuring a customer portfolio to reduce the vulnerability and volatility of cash flows. This article demonstrates how financial portfolio theory provides an organizing framework for (1) diagnosing the variability in a customer portfolio, (2) assessing the complementarity/similarity of market segments, (3) exploring market segment weights in an optimized portfolio, and (4) isolating the reward on variability that individual customers or segments provide. Using a seven-year series of customer data from a large business-to-business firm, the authors demonstrate how market segments can be characterized in terms of risk and return. Next, they identify the firm's efficient portfolio and test it against (1) its current portfolio and (2) a hypothetical profit maximization portfolio. Then, using forward-and back-testing, the authors show that the efficient portfolio has consistently lower variability than the existing customer mix and the profit maximization portfolio. The authors provide guidelines for incorporating a risk overlay into established customer management frameworks. The approach is especially well suited for business-to-business firms that serve market segments drawn from diverse sectors of the economy.Keywords: customer portfolio management, market-based assets, financial portfolio theory, return on marketing, market segmentation A lthough risk management is central to financial portfolio theory and occupies much of chief financial officers' time, researchers have given sparse attention to risk in the theory and practice of market segmentation and customer portfolio management. The existing portfolio of most firms reflects incremental and uncoordinated decisions from the past, when they gave little attention to how newly acquired customers contributed to the profitability and risk of the entire portfolio. For example, Homburg, Steiner, and Totzek (2009) find that firms tend to overestimate the value of top-tier customers and underestimate that of bottom-tier customers. In a similar vein, Dhar and Glazer (2003, p. 88) observe that few companies consider "whether all of their individually desirable customers are, from the standpoint of risk, desirable collectively." This practice is at odds with financial portfolio theory, which posits that although assets are selected individually, performance is measured on the entire portfolio, in which there is a tradeoff between risk and return (Markowitz 1952). We theorize that, like a financial portfolio, a customer portfolio is formed by making choices among market-based assets (i.e., customers) that present different risk-reward characteristics and allocating resources to optimize performance (Gupta and Lehmann 2005; Srivastava, Shervani, and Fahey 1998).The purpose of our research is to explore how financial principles of diversification and the tenets of financial portfolio theory can be effectively applied to manage a firm's customer portfolio. We demonstrate how fundamental tools of analysis that professio...