Vasopressin is a peptide hormone produced in the hypothalamus and released from the posterior pituitary. Secretion of vasopressin is followed by activation of its receptors V1a, V1b, and V2 throughout the body. Each receptor type is responsible for a specific function of vasopressin. For example, V1a receptor activation triggers vasoconstriction, V1b receptor is responsible for modulation of mood and behavior, and V2 receptor induces water reabsorption in the kidney. Vasopressin is known to regulate blood pressure, blood osmolality, and blood volume. The effects of V1a and V2 receptors can be amplified when vasopressin is secreted in excessive amounts, and this condition may be experienced by patients undergoing a disease or stress. In pathological conditions such as stroke, traumatic brain injury, subarachnoid hemorrhage, liver disease, and other diseases, vasopressin can exacerbate brain edema. Oversecretion of vasopressin unleashes deleterious pathways leading to hyponatremia and brain edema. This book chapter describes important mechanisms and pathways linking vasopressin and brain edema triggered by various conditions.