This chapter discusses the diagnosis, evaluation and management of tachyarrhythmias. Ironically, the classic presentation for tachyarrhythmias mostly consists of non-specific symptoms. Patients may complain of palpitations, chest pain, lightheadedness, dyspnea, or non-specific weakness. Further evaluation will reveal a rapid heart rate on physical examination or on the electrocardiogram (ECG). Patients with unstable tachyarrhythmias present with signs and symptoms of hypoperfusion and hemodynamic compromise while still maintaining a palpable pulse. Patients who do not have a palpable pulse are deemed to be in cardiac arrest and are treated according to Advanced Cardiovascular Life Support (ACLS) guidelines. Once tachyarrhythmia is confirmed, consideration should be given to whether the arrhythmia has an underlying noncardiac etiology such as a toxic ingestion or a metabolic disturbance. The primary goal with a patient in sinus tachycardia (ST) is to treat the underlying condition rather than the tachycardia itself.