Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in clinical practice and induces cardiac dysfunction and strokes. The development of AF requires a “trigger” and also an electroanatomic “substrate” capable of both initiating and perpetuating AF. Over the past decade, the understanding of the AF substrate properties in both atria has increased with fractionation and frequency analyses of the local atrial electrograms using three-dimensional electroanatomic mapping systems. The purpose of this review is to discuss the differences in the atrial substrate properties in patients with different types of AF.