My research approach is to combine science and engineering to understand and improve the performance, reliability, and security of Internet systems. With respect to science, I empirically measure computer systems that interact over the Internet to understand how well they match existing models and assumptions, then investigate the root causes for violations of those models/assumptions—often then leading to the design of new models. In many cases, our observations also suggest the design of systems that exploit previously unknown information about how our Internet-enabled systems work, and as an engineer I build and evaluate such systems in a way that other researchers, users, and policy makers can benefit from the result. To date, the software artifacts of my research have more than one million users, and my research teams have produced reports and datasets that informed additional research, policy debates, regulators, and legislators.