<p>Addiction is a complex, chronic disorder that affects tens of millions of people around the world. It is characterized by compulsive substance use or engagement in harmful behaviors despite adverse consequences. Recognized as a chronic and relapsing brain disorder, the transition from casual drug use to addiction involves a gradual process of escalated intake underpinned by significant changes in brain function, particularly in neural circuits governing reinforcement, learning, and behavior. This article is based on an assessment of the preclinical and clinical literature, delineating the biphasic nature of the addiction cycle—from use to abuse to addiction. The biphasic model focuses on progressive neurobehavioral adaptations that underlie phase transitions in stimulus reinforcement, motivation, associative learning, tolerance, and inhibitory control. Understanding how drug-induced alterations in brain function affect cognition, emotion, and behavior is essential for developing more effective strategies for addiction detection, recovery, and prevention.</p>