The relationship between endocardial anatomy and the substrate for a variety of atrial arrhythmia mechanisms is being increasingly appreciated. By using intravascular ultrasound imaging systems in the cardiac chambers, direct endocardial visualization can be provided. The advantages include: precise anatomic localization of the ablation catheter tip in relation to important endocardial structures that cannot be visualized with fluoroscopy; the ability to guide ablative procedures partly, or in some instances entirely, by anatomic landmarks; potential reduction in fluoroscopy time; evaluation of stability of catheter tip-tissue contact; confirmation of lesion formation and identification of lesion size and continuity; immediate identification of complications such as clot and pericardial effusion; assistance in the guidance of transseptal puncture; and as a research tool to help in understanding the critical role played by specific endocardial structures in atrial arrhythmogenesis. Presently, intracardiac echocardiography (ICE) is useful as an adjunct in guiding mapping and ablation of focal atrial tachycardia. In our laboratory, it has significant advantage in modification or ablation of sinus node function in patients with inappropriate sinus tachycardia syndrome. Its use in helping to guide ablation of atrial fibrillation remains an exciting, but largely unproved, hypothesis. Better technology will be required if widespread clinical use of ICE is to occur.