Patients with cardiac rhythm disturbances may present in a variety of conditions. Patients may be unstable, requiring immediate interventions, or stable, allowing for a more deliberate approach. Rapid assessment of patient stability, underlying rhythm, and determination of appropriate interventions guides timely therapy. This article discusses the differential diagnosis and treatment of adult patients presenting with primary bradyarrhythmias and tachyarrhythmias, with the exception of atrial fibrillation and atrial flutter, covered elsewhere in this issue. A concise approach to diagnosis and determination of appropriate therapy is presented.