Online markets for child exploitation materials (CEM) or child sex abuse images have developed in tandem with the rapid expansion of the Internet. CEM images of sexual abuse of children, often photographs and videos, include live transmission of sexual assault as well as simulated experiences using virtual reality technology or robotics. User-generated images, sexting and ‘revenge porn’ are new sources of image-based abuse often involving older children. The content of CEM constitutes a crime scene that can help trace offenders and identify victims, but poses challenges for digital forensics. This chapter describes the prevalence and nature of CEM, profiles of offenders and victims, and law enforcement responses. Discussed are definitions of CEM; the relationship between online and offline or contact offending; emerging developments in CEM; the role of transnational policing; and international collaborative and regulatory approaches to suppress CEM.