Addictive behavior developes after repeated substance use and it typically include a strong desire to take the drug, difficulties in controlling its use, persisting in its use despite harmful consequences, a higher priority given to the drug use than to other activities. Relapse, the resumption of drug taking after periods of abstinence, remains the major problem for the treatment of addiction. The process of drug addiction shares striking commonalities with neural plasticity associated with natural reward learning and memory and is caused primarily by drug-induced sensitization in the brain mesocorticolimbic systems that attribute incentive salience to reward-associated stimuli. The switch from controlled to compulsive drug seeking represents a transition at the neural level from prefrontal cortical to striatal control. Current neurophysiologic evidence suggests that the development of addiction is to some extent due to neurochemical stimulation of the midbrain dopaminergic system that is traditionally considered as a 'common neural currency' for rewards of most kinds. Addictions are a result of the interplay of multiple genetic and environmental factors. They are characterized by phenotypic and genetic heterogeneity as well as polygenicity. Environmental factors are crucial in addiction vulnerability and resistese too.