Tachyarrhythmias can present a significant clinical challenge. Patients presenting with fast heart rate may be unstable with chest pain, hypotension, or with myocardial ischemia, or they may be completely asymptomatic. The initial approach typically involves distinguishing between tachycardias with a wide QRS complex (“wide complex tachycardias”) versus those with a narrow QRS complex (“narrow complex tachycardias”). Narrow complex tachycardias are always supra-ventricular in origin. Wide complex tachycardias are more complex as they may arise from supra-ventricular sources and conduct aberrantly (supra-ventricular tachycardia with aberrancy) or they may arise from ventricular sources (ventricular tachycardia). In this case, we will examine a patient presenting with tachycardia and work our way through a differential diagnosis.