This is a sequential phase 1 and phase 2 study to evaluate the efficacy of intracardiac echocardiography to detect septal and left atrial pathology as compared to transesophageal echocardiography (Phase 1) and its value in a management strategy for immediate cardioversion during cardiac catheterization procedures in patients with atrial fibrillation as compared to a conventional strategy delaying cardioversion till full anticoagulation is established for a three weeks (Phase 2). Phase 1 will enroll 100 patients at 12 centers; these patients will be undergoing clinically indicated TEE & cardiac catheterization procedures. After review of Phase 1 results by an independent DSMB & the investigators that establish efficacy of ICE, Phase 2 will be initiated. Phase 2 will enroll 300 patients in 15 centers; these patients with atrial fibrillation will be undergoing clinically indicated cardiac catheterization procedures and have a clinical indication for cardioversion. Patients will be randomized to ICE guided cardioversion strategy or a conventional strategy employing three weeks of full anticoagulation before cardioversion. ICE imaging will be used to identify a low risk group for immediate cardioversion. A composite primary study endpoint that will include mortality and major morbidity including stroke and bleeding complications will be used.