From many years of research, we have come to the conculsion that, except in persons on very high intakes of salt, one can develop hypertension only when the kidenys function abnormally (1). The abnormal function might result from intrinsic changes within the kidneys themselves or from the effect of nerve signals, hormones, or other factors acting on the kidneys. The goal of this paper is to explain briefly why we have come to this conclusion. When most persons think of the kidneys and hypertension, they think of the renin-angiotensin system and its effect to increase the blood pressure. However, it will quickly become apparent that our research has implicated even more the role of the kidneys to control body fluid volume which in turn controls the long-term level of arterial pressure as well. Therefore, we will divide this paper into two major parts: (1) The effect of changes in body fluid volume on the long-term level of arterial pressure, and (2) the reciprocal effect of arterial pressure on the kidneys to make the kidneys eliminate fluid volume. Then we will show how the balance of these two factors is the principal determinant of the long-term level of arterial pressure.