There are two major categories of hydrops fetalis: (i) Immune hydrops (IH)—due to maternal hemolytic antibodies. (ii) Nonimmune hydrops (NIH)—due to all other etiologies. In this chapter only nonimmune hydrops is discussed in brief. Hydrops Fetalis describes a fetus or neonate with a pathological increase in total and interstitial body water, manifested as generalized subcutaneous edema, accompanied by serous fluid collections within one or more of the pericardial, pleural, or peritoneal spaces. Although there have been many advances in our understanding of the causes of fetal NIH, it remains a difficult clinical problem. Once the diagnosis of NIH is established, a careful search for causative fetal pathology should be undertaken. Hydrops fetalis indicates fetal compromise in utero, fetal outcome of serious morbidity or mortality will depend upon the actual cause of the fetal hydrops. The antenatal detection of fetal hydrops requires an expeditious and diligent search for the underlying cause in order to direct appropriate pregnancy counseling and management. Unfortunately, the results of such a search may not be available when difficult management decisions need to be made. Recent advances in fetal therapy have increased the number of fetal conditions for which treatment is possible. However, the overall rates of morbidity in mother and fetus, and of mortality in the fetus, remain high.