Addiction is a state characterized by compulsive engagement in rewarding stimuli, despite adverse consequences. The process of developing an addiction occurs through instrumental learning, which is otherwise known as operant conditioning. The nucleus accumbens (NAc) plays an essential part in the formation of addiction. Almost every drug with addictive potential induces the release of dopamine into the NAc. In contrast to the VTA, the NAc shows long-term structural changes. Drugs of abuse weaken the connections within the NAc after habitual use, as well as after use then withdrawal.