**Abstract**

This chapter will review the altered brain structure and function associated to drug addiction, with a focus on brain regions involved in learning and motivated behavior. As evidenced by both clinical and preclinical studies, repeated drug exposure affects whole brain neuroplasticity including the mesolimbic system which is a main locus for reward, an action-control center such as the dorsal striatum, and limbic brain regions such as the prefrontal cortex, the hippocampus, and the amygdala that are involved in behavioral control, memory, and mood. In this way, the drug-seeking actions that were initially intentional responses become involuntary habits governed by the dorsal striatum. Drug addiction may also curse with a reduced ability to experience rewards that are unrelated to drugs and emotional dysregulation, while the impairment on limbic regions contributes to generate cognitive symptoms. These entail persistent memories for previous experiences with the drug contrasting with a global cognitive decline that may hamper the acquisition of new, adaptive learnings. Overall, these features promote a desire for the drug, leading to relapse in drug use. Further drug exposure, in turn, aggravates its consequences on the brain and behavior, creating the harmful "addiction cycle."

**Keywords:** substance use disorders, habits, motivation, memory, mood, accumbens, striatum, limbic regions
