Premature atrial complexes (PACs) are among the most common arrhythmias encountered in daily practice and are often dismissed as benign. But are they truly harmless—or could some represent the earliest stage of atrial fibrillation (AF)? Hosts Mitsuaki Sawano, MD, and Hiroyuki Sato, MD welcome Satoshi Higuchi, MD (Tokyo Women's Medical University) to discuss his JACC: Clinical Electrophysiology study, Atrial Effective Refractory Periods in Patients With Frequent Premature Atrial Complexes: Implications for Atrial Fibrillation. Building on translational work from experimental models, the study compares atrial effective refractory periods (AERP) in patients with frequent PACs, paroxysmal AF, and persistent AF to better define the electrophysiologic substrate underlying these conditions. The episode explores how electrical remodeling differs between PACs and AF, why pulmonary vein AERP shortens before more diffuse atrial remodeling develops, and what these findings reveal about the progression from atrial ectopy to sustained arrhythmia. The discussion also examines the emerging concept of PAC-induced atrial cardiomyopathy, the challenge of distinguishing "benign" from potentially malignant PACs, and the future of risk stratification and preventive intervention before AF becomes established.