The heart of the public library mission is to provide service to the entire community and serve the underserved. But how does a library identify the people it is not reaching, where they live, and what they need? And what does it mean for a library's services if in its community the underserved are not also underprivileged? Contrary to popular belief, in a surprising number of communities the upwardly mobile middle and upper-middle classes are underserved. This creates a marginalized library with eroding support. In this article, the author explains how market segmentation can help librarians in identifying their underserved populations.