Established in 1999, the Drug Use Monitoring in Australia (DUMA) program is funded by the Australian Government and is the nation’s longest-running ongoing survey of police detainees across the country. DUMA comprises two core components: a self-report survey on drug use, criminal justice history and demographic information; and voluntary urinalysis, which provides an objective measure for corroborating reported recent drug use. This report describes drug use, drug market participation and the extent to which detainees’ alleged offences were related to drug or alcohol use. Data were collected between January and December 2018 at five sites: Adelaide, Brisbane, Perth, Bankstown and Surry Hills. Since police detainees are more likely than the general population to have been in recent contact with the illicit drug market, understanding their drug use and offending habits is valuable in the formulation of policy and programs.