This paper discusses the use of web metrics tools at four digital libraries, the Instructional Architect, the Library of Congress, the National Science Digital Library, and WGBH Teachers' Domain. We describe some of the issues involved in using web metrics to report on ongoing web site performance. We also describe how web metrics can be used for focused data mining, using session length metrics as our example; and here, we recommend that session length metrics, which were developed to track e-commerce, need to be carefully considered when they are applied in non-e-commerce settings, such as digital libraries. We conclude by discussing some of the current limitations and possibilities of using web metrics to analyze and evaluate digital library use and impact.