Illicit activities merit particular consideration not only because of the importance of the issues at stake, but because of some unique features requiring special attention. Data is hard to collect and hence hard to evaluate. Many participants naturally maximize secrecy, evading tracking and monitoring and systematic study. Those who can quantify some aspects of illicit activity may conceal the resulting data; law enforcement officials keep some information secret for investigative and enforcement purposes, groups assisting victims may need to keep the identity (or even the existence) of the victims secret for protection; one institution may mistrust another. Yet, even in contexts where information is impervious to public scrutiny, the power-knowledge effects of (accurate or inaccurate) data may be great. Examining how the data is produced, analyzed, and used and how data producers, users, and other actors interact in the context of monitoring and enforcing illegal and illicit activities opens up new perspectives on the roles of quantification and of multi-institutional dynamics in governance. Bringing together experts on these issues and experts on several different kinds of illicit activity allows for comparative study of governance, data, and measurement in domains that are characterized by different degrees of transparency and different institutional dynamics. Policy and practical innovation, and cross-issue linkage, may also be facilitated.