Vasopressors are agents that increase systemic vascular resistance by increasing vasoconstriction. Therapy with intravenous vasopressors may be required in critically ill patients when efforts to optimize cardiac output and blood pressure with intravascular fluid therapy fail. Increasing systemic vascular resistance can promote a favorable perfusion pressure gradient to vital organs in critically ill patients with severe, unresponsive vasodilation. Improperly administered, vasopressors may impede cardiac output and reduce oxygen transport to vital tissue sites. The understanding of systemic and regional effects of vasopressors is currently evolving. Recent literature of the commonly used agents is reviewed. Individual drugs, drug combinations, and potential new therapies are discussed. (Vet. Emerg. & Crit. Care, 10:19–33, 2000)