A hypertensive crisis is an abrupt and severe rise in the arterial blood pressure (BP) occurring either in patients with known essential or secondary hypertension, or it may develop spontaneously. The most frequent cause for the severe and sudden increase in BP is inadequate dosing or stopping antihypertensive treatment in hypertensive patients. Severe hypertension can be defined as either a hypertensive emergency or an urgency, depending on the existence of organ damage. In hypertensive urgencies, there are no signs of acute end-organ damage, and orally administered drugs might be sufficient. In hypertensive emergencies, signs of acute end-organ damage are present, and in these cases, quickly-acting parenteral drugs must be used. The prompt recognition, assessment, and treatment of hypertensive urgencies and emergencies can decrease target organ damage and mortality. In this review, the definitions and therapeutic recommendations in a hypertensive crisis are presented in the light of the 2017 ACC/AHA Hypertension Guidelines.