Fibrosis is an essential component of structural remodeling that accompanies different cardiovascular disease conditions. In cardiac arrhythmias, fibrosis plays an important role in creating the necessary substrate for these arrhythmias to persist. Non-invasive evaluation of the arrhythmia substrate has seen significant advancements in the past several years. Cardiac MRI characterizes tissue changes including fibrosis and scarring and has seen its role expand significantly in cardiac electrophysiology. We have used late-gadolinium enhancement MRI (LGE-MRI) to study tissue changes in the left atrium of patients with atrial fibrillation (AF) and demonstrated its utility in identifying poor responders to catheter ablation, as well as its association with stroke risk and significant sinus node dysfunction. Following ablation, we also studied atrial scarring and demonstrated the utility of scar imaging in guiding repeat ablation procedures.