Long-term intake of a psychoactive drug alters brain signal transduction, emotional and motivational factors and behavioral parameters. Some effects that outlast long periods of abstinence are due to the long-term presence of the drug in the organism (tolerance, physical dependence). Withdrawal symptoms, as a consequence of physical dependence, can be protracted, i.e. they persist after long periods of drug deprivation (e.g. a desensitization of the production of cAMP). Further persisting effects include experience-based learning. At least three distinct processes can be differentiated: a memory of drug effects (reflected by a sensitization to drug effects etc.), a memory of drug use (reflected by controlled drug consumption), and a memory of addiction (reflected by a persisting loss of control over drug intake and correlating changes in striatal dopaminergic neurotransmission). The latter probably consists of two components: a general memory of loss of control and a specific memory of the addictive drug (general principles for the development of addiction, specific of the urge for the addictive drug).