Atrial fibrillation (AF) is the most important arrhythmia in contemporary cardiology, and despite much progress, remain therapeutic challenges. Invasive electrical studies of the heart are often used in the diagnosis and therapy of arrhythmia. Many forms of arrhythmia can be cured by selective destruction of critical electrical pathways with catheter ablation. A major limitation in studying arrhythmia, however, is the inability to visualize soft tissues, and ablated areas of myocardium during catheter ablation procedures. After performing extensive preliminary work on MRI guided electrophysiology in animal models, the aim of this study is to examine the safety and feasibility of real-time MRI guidance for clinical electrophysiology studies and cardiac ablation procedures using custom made catheters, radiofrequency filters, and a shielded signal acquisition system. The objective is to test the hypothesis that MRI guidance will improve the safety and efficacy of clinical ablation procedures. Specific Aim 1) To study the determinants of successful ablation in patients undergoing standard fluoroscopy guided ablations. Patients with AF will undergo electrophysiology study and ablation using standard fluoroscopy guidance. Completeness of ablation lines will be assessed by MRI, and correlated with acute and chronic recurrence of AF. The effect of ablation lesions on atrial re-modeling will be assessed at 6 months. This aim will provide insight into determinants of success and complications post ablation. Specific Aim 2) To study real-time MRI guidance of electrophysiologic studies and catheter ablation in patients with AF. This portion of the study will be conducted in participants with symptomatic AF. The primary endpoint will be lack of AF documented by Holter monitoring 6 months post ablation. The secondary endpoints will be comparison of inducible arrhythmia at the end of the procedure, procedure time, and complications in each study arm.