Purpose of Review <p>This narrative review synthesizes research on transdiagnostic etiological mechanisms underlying attention-deficit/hyperactivity disorder (ADHD) and alcohol use disorder (AUD).</p> Recent Findings <p>Findings converge on three overarching transdiagnostic domains contributing to both ADHD and AUD: impulsivity, reward-related dysfunction, and attentional control. Within these domains, several specific mechanisms predominate as especially robust risk factors, including state-level emotional impulsivity (positive and negative urgency) and delay and probability discounting. Findings emphasize the utility of real-time, ecological interventions targeting these mechanisms. Our broad survey of the ADHD and AUD literatures reveals substantial overlap and synergy among these transdiagnostic mechanisms, highlighting the need for definitional precision and the joint examination of these constructs.</p> Summary <p> Though the transdiagnostic mechanisms linking ADHD and AUD are complex, focusing on impulsivity, reward-related dysfunction, and attentional control in future research and intervention development is likely to maximize scientific advances and translational impact for individuals experiencing these prevalent and burdensome disorders.</p>