This paper reviews the current state of knowledge on illicit drugs as emerging environmental contaminants. Several studies have recently reported that illicit drugs are detectable in wastewater from municipal sewage treatment plants (STPs) and surface waters. These substances are excreted in urine and faeces unchanged or as active metabolites in high percentages after consumption and continuously discharged into domestic wastewaters. Residues of illicit drugs can therefore reach STPs in substantial amounts, escaping degradation, and are then released into surface waters. Environmental concentrations are low, but risks for human health and the environment cannot be excluded. Morphine, cocaine, methamphetamine and ecstasy all have potent pharmacological activities, and their presence as complex mixtures in surface waters may be toxic to aquatic organisms. Levels of residues in untreated wastewater have been used to estimate illicit drug consumption in the population. Given that current epidemiological methods are indirect and possibly biased, this evidence-based approach offers a new tool for estimating drug abuse in real time.