Illicit drugs were recently indicated as emerging contaminants since they have been detected in waste, surface and drinking water and in the airborne particulates in several European countries and USA. In analogy with pharmaceuticals, the main source of contamination for illicit drugs is human consumption. The residues of drugs of abuse persisting in consumers' urine can reach sewage treatment plants (STPs) in detectable amounts, escaping degradation, and can be released into surface water. The first investigations of illicit drugs in the environment were carried out in U.S.A. in 2004 for amphetamines, and in Italy in 2005 for cocaine and its main urinary metabolite (benzoylecgonine, BE). Several other substances were later measured in water and air, including cannabinoids, cocaine and its metabolites, opioids, amphetamines, ephedrine, ketamine, lysergic acid diethylamide (LSD), and some related opioid pharmaceuticals. The first step to measure illicit drugs in the environment consists in the preconcentration of analytes operated mainly by solid phase extraction (SPE). Considering the complexity of the environmental matrices and the low concentrations of the analytes, mass spectrometry is the most powerful technique to detect illicit drugs simultaneously with high specificity and accuracy. Therefore, the technique used most frequently is high-pressure liquid chromatography tandem mass spectrometry (HPLC-MS-MS). Illicit drugs were frequently detected at concentrations up to the μg/L range in STP influents (untreated wastewater) in Europe and U.S.A. Cocaine and its major metabolite BE were investigated in the largest number of countries (Spain, Italy, Switzerland, UK, Belgium, Germany, Ireland and U.S.A.), and were the most abundant compounds. Other substances measured at high concentrations were several stimulatory drugs, including amphetamines and ephedrine, opioids, and the main metabolite of cannabis, 11-nor-9-carboxy- δ9-THC (THC-COOH). Despite the fact that illicit drugs are generally well removed in STPs (removal higher than 60%), several substances were still detected at concentrations up to the hundreds of ng/L in STPs effluents. Treated wastewater is generally discharged into surface water (rivers, lakes, sea) or undergoes further treatment to produce drinking water. Substantial amounts of illicit drugs therefore direclty enter surface water or drinking water treatment plants. These substances were still detectable in rivers and lakes up to tenths of ng/L in several countries, and trace amounts of BE, methadone and its main metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), were still present in finished drinking water in a Spanish drinking water treatment plant. Traces of illicit drugs were also detected in airborne particulate in several of the world's cities, indicating the possible distribution of these substances in the air compartment, despite their polarity and high water solubility. The amounts of illicit drugs detected in wastewater could roughly reflect the amounts consumed. Our group recently proposed a novel approach (sewage epidemiology) for estimating drug consumption in a community by the direct measurement of the residues of the illicit drugs in urban wastewater. This method can give evidence-based estimates of drug use in a defined area with the unique ability to monitor local consumption in real time and promptly identify changes..