Cardiac electrophysiology is a clinical discipline that aims to diagnose and treat cardiac arrhythmias. There are a variety of tools and techniques that may be used to evaluate arrhythmias. In this chapter, we discuss applications of CT and MRI in guiding electrophysiological procedures with a focus on quantitative imaging approaches. Modern quantitative imaging techniques allow for precise localization and quantification of arrhythmogenic substrate that may assist the electrophysiologist in planning and executing ablation procedures as well as in evaluating procedure outcomes. Topics discussed include ablation for atrial fibrillation (AF), ablation for ventricular tachycardia (VT), planning for cardiac resynchronization therapy (CRT), and emerging technologies for analyzing arrhythmias and predicting treatment responses. For AF ablation, CT allows for accurate delineation of anatomical structures including measurements such as pulmonary ostia sizes and peri-esophageal fat pad thickness, and MR allows for localization and quantification of left atrial fibrosis. For VT ablation, both MRI and CT can be used to localize and quantify arrhythmogenic substrate in the form of fibrotic scar, and this information may assist the electrophysiologist in planning an optimal approach and dose for ablation. For CRT planning, functional information from MRI may help to identify patients who may benefit from CRT, and anatomical information from CT or MRI may assist in planning access. Finally, emerging technologies such as patient-specific computational models may serve as a virtual laboratory that may be used to analyze and predict the effects of antiarrhythmic medications or ablations on an individual patient’s electrophysiology.