The main illicit markets for heroin, cocaine, cannabis, and amphetamine-type stimulants (ATS) are analyzed for the period between 1998 and 2003. The illicit drug production, trafficking, and consumption trends focus on 2001/2002. The evolution of the main illicit drug markets in recent years is inconsistent depending on the drugs and the regions considered. The evolution of world heroin and cocaine markets shows some positive tendencies. The picture is confused for synthetic drugs and fairly negative for cannabis. Treatment data consistently show that illicit heroin use has the most severe health consequences for drug abusers. Cocaine is abused by about 14 million people in the world and comes second to heroin in terms of treatment demand. Cannabis continues to be the most widely produced, trafficked, and consumed illicit drug worldwide. Amphetamine-type-stimulants (ATS) are synthetic drugs that include the chemically related amphetamine, methamphetamine, and ecstasy. Seizures of laboratories and end products, as well as reports of consumption, indicate that the expansion of the ATS market is continuing. Trafficking in methamphetamine accounts for the bulk of trafficking in ATS. After the steep decline recorded in 2001, world illicit opium and heroin production recovered in 2002. Overall trafficking, as reflected in the number of seizure cases reported, continued growing in 2001, though at a slower pace than in the 1990's. About 200 million people consume illicit drugs, including about 163 million for cannabis, 34 million for amphetamines, 8 million for ecstasy, 14 million for cocaine, and 15 million for opiates (of which 10 million for heroin). These numbers are not cumulative because of polydrug use. Overall, the global drug problem continues to spread in geographical terms as more countries report increases rather than decreases in drug abuse.