As with any disease, vulnerability to addiction differs from person to person. In general, the more environmental and genetic risk factors an individual has, the greater the chance that taking drugs will lead to abuse and addiction. We are interested in whether the functioning of the mesocorticolimbic dopamine system is predictive of drug reward, drug taking and/or relapse. One line of research is investigating how dopamine D2 receptor sensitivity may predict addiction vulnerability. Studies show that drug addicts tend to have a reduced availability of dopamine D2 receptors, however it is unclear whether reduced availability of dopamine D2 receptors is a vulnerability factor or a consequence of repeated drug intake. It was recently suggested that non-abusers with low levels of dopamine D2 receptors report more pleasant experiences when taking drugs of abuse. Thus, dopamine D2 receptor levels and/or sensitivity may predict future drug use. We have been investigating this idea by exploiting individual differences in the behavioral locomotor effects resulting from dopamine D2 receptor stimulation. Early studies suggest that increased dopamine D2 receptor sensitivity predicts enhanced stimulant and rewarding properties of psychostimulants. Another line of research in our lab is designed to identify environmental factors that may contribute to subsequent vulnerability to drug addiction. We are currently exploring the effects of caffeine consumption during adolescence on several indices of psychostimulant addiction (reward, self-administration and relapse). Data suggests that adolescent caffeine exposure enhances the stimulant and rewarding properties of cocaine and that these enhancements may be due to caffeine-induced increases in the sensitivity of the dopamine D2 receptor. We are currently exploring the neurobiological changes in the mesocorticolimbic system resulting from chronic caffeine exposure in adolescents that persist into adulthood.