Atrial fibrosis is thought to increase propensity for atrial fibrillation (AF). We tested the hypothesis that fibroblast proliferation and myocyte-fibroblast coupling significantly alter electrical wave propagation and reentry dynamics. Myocyte-myofibroblast Co-cultures from neonatal rat ventricles were optically mapped using a voltage-sensitive dye during pacing and sustained reentry. The myofibroblast/myocyte ratio was changed systematically, and junctional coupling was reduced using Cx43 RNA silencer, or increased using overexpression by adenoviral transfer in the myofibroblasts. Reentry frequency and conduction velocity (CV) diminished with larger myofibroblast/myocyte area ratios; complexity of propagation increased, resulting in wave fractionation and reentry multiplication. The relationship between CV and myocyte-fibroblast coupling was biphasic. Thus, monolayer data predicted that replacement fibrosis should alter CV and complexity of propagation during AF. Hence, we test such predictions, using a chronic sheep heart failure (HF) model to study AF dynamics. Optical mapping of the posterior left atrium (PLA) demonstrated that HF reduces AF frequency. However, during AF wave propagation patterns are more variable and complex in HF than control. In HF such variability correlates with the architecture of fibrosis in the PLA, which consists primarily of large patches forming obstacles preferentially in the vicinity of the pulmonary vein ostia. In experiments and in computer simulations, large fibrous patches acted to anchor rotors and slow their frequency, as well as to impair wave propagation, generating delays, wavebreaks, rotor multiplication and wave fractionation. We concluded that the spatial distribution of patchy fibrosis governs wave propagation dynamics in the PLA and contributes to AF permanence.