The brain is composed of many different regions and structures. The brain's communication system permits these various regions and structures to coordinate their activities. Each of these different regions and structures serves different purposes. Addictions can alter these regions and structures. Subsequently, addictions can alter the way brain regions function. In this section, we discuss the regions and structures that are affected by the addictive process. We will review the brain's role in commonly observed problems associated with addiction: 1) Impaired decision-making, impulsivity, and compulsivity, 2) drug-seeking and cravings, 3) habit formation, craving, withdrawal effects, and relapse triggers and 4) stress regulation and withdrawal.