The Global Drug Survey (GDS) is a detailed annual online survey of psychoactive substance use that first ran in 2011. The 10 GDS surveys to date have collected data from more than 900 000 respondents, containing information that can answer key and novel questions related to the epidemiology of psychoactive substance use as well as inform the development of pragmatic harm reduction resources that put people first. This chapter looks at the history of the GDS and the range of harm reduction tools that have been created using data from the survey, highlighting the importance of disseminating peer expertise to inform positive behaviour change. This includes several mobile health applications developed from GDS data, aiming to dismantle barriers to behaviour change by considering individual risk factors. For example, the Drugs Meter tool allows people to compare their last month’s use of cannabis, cocaine or MDMA to those of tens of thousands of other people, enabling people who use drugs to become involved in a dialogue about their use. This highlights the potential of using data collected from online surveys to develop practical online tools for promoting harm reduction among people who use drugs.