The behavior of the kidney in normal pregnancy, as well as in complicated pregnancy, is a very interesting, but still in many ways an unknown topic in renal medicine. It is undoubtedly difficult to determine, even in normal women, the behavior of renal hemodynamics throughout gestation, since the fear of impairing a new life (i.e., the fetus''s life) will limit, for ethical reasons, the use or the frequent repetition of diagnostic tests on the mother. On the other hand, the study of complicated pregnancy even for diagnostic purposes (for planning adequate treatment), except in a few countries that are known for the advanced health education of the population, has to face serious difficulties. First of all, pregnant women usually seek the help of an obstetrician when gestation is already in an advanced stage. This makes it difficult to determine when and how asymptomatic signs of any disease discovered during pregnancy have first occurred. A second difficulty is that frequently the patient does not know whether a given disease has preceded pregnancy. Pregnancy is a condition of young women, and a young woman frequently has never seen a physician; thus, no urine analysis or blood tests have been performed before the gestation. Not infrequently, even blood pressure has never been measured. This will make it difficult to classify hypertension discovered in late pregnancy as pregnancy-induced hypertension or as chronic hypertension in pregnancy.