Assessing and monitoring the performance of computer and communications networks is an important problem for network engineers. A considerable amount of work has been done on tools and techniques for data collection, modeling, and analysis within the network research community. This article presents an overview of the engineering problems and statistical issues, describes recent research developments, and summarizes ongoing work and areas for further research. Although there are many interesting issues related to network analysis, our focus here is on estimating and monitoring network quality-of-service parameters. We discuss methods for estimating edge-level parameters from end-to-end path-level measurements, an important engineering problem that raises interesting statistical modeling issues. Other topics include network monitoring, network visualization, and discovery of network topology. Data from a corporate network are used to illustrate the problems and techniques. As in any overview, the discussion is likely to be slanted toward the authors' own research interests.