Atrial fibrillation (AF) is the most common arrhythmia and is associated with significant morbidity and mortality. Invasive catheter ablation of AF has emerged as an effective therapy for patients with symptomatic AF. Atrial remodeling is important not only for AF persistence, but also for AF recurrence after ablation. Atrial dilation and fibrosis are two of the core processes involved in atrial remodeling. Increased automaticity and triggered activity occur in atrial remodeling, which may result in difficulty in achieving AF extinction after ablation. Furthermore, an enlarged left atrium (LA) may increase the difficulty of achieving catheter stability and thereby require more energy to complete AF extinction. The occurrence of AF causes similar remodeling in both the left and right atria (RA), and myocardial changes in both atria influence AF recurrence. Non-invasive assessment of fibrotic structural remodeling helps to predict the outcome of AF ablation. Non-invasively measured atrial volumes are used to estimate atrial remodeling before AF ablation. A variety of cardiac imaging modalities, such as two- or three-dimensional echocardiography or multi-detector row computed tomography, have been used to estimate the extent of atrial remodeling by the measurement of atrial volume or of LA deformation. Furthermore, delayed enhanced cardiac magnetic resonance imaging has been used to detect atrial fibrosis. Thus, atrial remodeling plays an important role in AF recurrence, and these non-invasive imaging modalities are significant tools for estimating atrial enlargement and atrial fibrosis to improve patient selection for AF ablation.