The 12-lead electrocardiogram (ECG) is an invaluable tool for the diagnosis of supraventricular tachycardia (SVT). Most forms of SVT can be distinguished with a high degree of certainty based on specific ECG characteristics by using a systematic, stepwise approach. This article provides a general framework with which to approach an ECG during SVT by describing the salient characteristics, ECG findings, and underlying electroanatomical relationships of each specific type of SVT encountered in adults. It concludes by providing a systematic algorithm for diagnosing SVT based on the findings of the 12-lead ECG.