Risk stratification for stroke in individuals with atrial fibrillation (AF) is among the most challenging risk assessment problems in medicine. The disease is not binary, and measures of AF severity (burden, duration, or frequency) can rapidly fluctuate. Substrate markers associated with stroke (eg, left atrial size/structure/function, appendage emptying, or ejection fraction) may also fluctuate or be risk markers rather than causal factors. Ischemic stroke may not be from cardioembolism, even when AF is present.