There is significant experimental evidence implicating the endogenous opioid system (opioid peptides and opioid receptors) with the processes of reward and reinforcement. Indeed, many behaviors associated with reward and reinforcement, for example feeding behavior, are controlled by distinct components of the endogenous opioid system located in relevant brain regions. It has also been shown that regardless of their initial site of action many drugs of abuse, such as morphine, nicotine, cocaine, alcohol and amphetamines, induce an increase in the extracellular concentration of dopamine in the nucleus accumbens. This increased secretion of dopamine in the nucleus accumbens seems to be a common effect of many drugs of abuse, and it was proposed that may mediate their rewarding and reinforcing properties. Furthermore, activation of μ opioid receptors in the ventral tegmental area, or of μ and δ opioid receptors in the nucleus accumbens enhances the extracellular concentration of dopamine in the nucleus accumbens. Thus, stimulation of the activity of distinct components of the endogenous opioid system either by opioid or by other drugs of abuse, may mediate some of their reinforcing effects. In this review article, a brief description of the endogenous opioid system and its implication in the processes of reward and reinforcement of opioid and other drugs of abuse will be presented. Furthermore, the use of opioid antagonists in the treatment of drug addiction will be discussed. Special emphasis will be given to ethanol addiction, the drug mainly studied in my laboratory.