In the vast majority of patients, hypertension has no identifiable cause. Moreover, despite decades of discovery and therapeutic development, approximately only half of the hypertensive patients are achieving clinically recommended blood pressure control. For these reasons, there has been continued interest in studying the factors that underlie the pathogenesis of hypertension. One area of research that has garnered significant attention is the potential link between immune system activation and blood pressure. Several lines of evidence, dating back several decades, support the concept for this association. This monograph gives a history of the early studies linking immune system function with hypertension and an overview of the large number of studies published in the past decade. The major focus is on the components of the innate and adaptive immune systems for which there is considerable evidence of their contributions to blood pressure control.