The aim of this chapter is to explore the research opportunities afforded by the Internet. To do so, this chapter will describe both the indirect and direct monitoring techniques used to monitor the traces offenders leave online. Three case studies, on prostitution, financial fraud and online drug dealing, will demonstrate the potential of online monitoring. The online monitoring of offenders, both indirect and direct,arguably offers one of the most interesting opportunities for criminologists and forensic scientists to collaborate with one another. Forensic scientists are increasingly developing their expertise in the collection and analysis of digital traces. Criminologists, on the other end, have developed methods and research design to understand offending behaviour even when very limited data are available. Through online monitoring, criminologists are likely to gain a much deeper insight into offending trajectories, including who offenders are, how they become offenders, how they operate and how they desist from crime. Using data collected by forensic scientists, criminologists may be able to answer—or shine a new light on—questions that have plagued criminology for quite some time.