One-to-one marketing advocates tailoring of one or more aspects of the firm's marketing mix to the individual customer (Peppers and Rogers 1997; Peppers, Rogers and Dorf 1999; Shaffer and Zhang 2002). One-to-one marketing represents an extreme form of segmentation, with a target segment of size one. There are two forms of one-to-one marketing: personalization and customization. Personalization is when the firm decides, usually based on previously collected customer data, what marketing mix is suitable for the individual. A good example is Amazon.com's personalized book and music recommendations (Nunes and Kambil 2001). The e-commerce arena is replete with other instances of personalization. Nytimes.com allows readers to get personalized news articles of interest, MLS.ca in Canada screens houses for buyers depending on their preferences for location, size and features. Customization is when the customer proactively specifies one or more elements of his or her marketing mix. Dell computer allows customers to customize the computer they order. The MyYahoo feature at Yahoo.com allows users to specify elements of their home page such as the weather forecast, reports on their favorite stocks, or priorities given to local sports news. The purpose of this paper is to summarize key challenges and knowledge gaps in understanding the choices that both firms and customers make in a personalization/customization environment. We start with a summary of personalization and customization in practice, and then draw on research in economics, statistical, and consumer behavior to identify what we know and do not know. We conclude with a summary of key research opportunities.