Theoretical and experimental studies have shown that restitution of the cardiac action potential duration (APD) plays a major role in predisposing ventricular tachycardia to degenerate to ventricular fibrillation, whereas its role in atrial fibrillation is unclear. We used the Courtemanche human atrial cell model and Luo-Rudy guinea pig ventricular models to compare the roles of electrical restitution in destabilizing spiral wave reentry in simulated 2-D homogeneous atrial and ventricular tissue. Because atrial action potential morphology is complex, we also validated the usefulness of effective refractory period (ERP) restitution. ERP restitution correlated best with APD_-62mV restitution, and its steepness was a reliable predictor of spiral wave phenotype (stable, meandering, hyper-meandering, and breakup) in both atrial and ventricular tissue. Spiral breakup or single hyper-meandering spirals occurred when the slope of ERP restitution exceeded 1 at short diastolic intervals. Thus ERP restitution, which is easier to measure clinically than APD restitution, is a reliable determinant of spiral wave stability in simulated atrial and ventricular tissue.