The online space has emerged as a promising avenue for conducting surveys and identifying trends in illicit drug use, as information can be collected quickly and cheaply from large numbers of people who use drugs, providing an opportunity to fill important knowledge gaps about how drugs are used. This study reviews the existing literature around online survey methods and their specific application in the area of drug use, looking at their main advantages, drawbacks, limitations and areas for further development. By providing respondents with a sense of anonymity and privacy, web surveys may be particularly useful for obtaining information on sensitive topics like drug use. Drawbacks of web surveys include sometimes having high levels of item non-response and incompleteness, and that respondents may be prone to ‘satisficing bias’. One of the main limitations of these surveys is that they are usually not representative of the general population or of any pre-defined populations of people who use drugs. Nevertheless, self-selected online drug surveys remain an important tool for collecting data from hidden populations of people who use drugs. They can provide in-depth insights into patterns, practices and features of drug use and drug markets, and the relationships between these variables.