Hydrops fetalis, meaning "edema of the fetus," is a clinicopathological term describing an end-stage status of fetal fl uid imbalance, and the diagnostic criteria include generalized skin edema with one or more serous cavity effusions. The underlying causes include various maternal, fetal, and placental conditions. Even if the underlying forces to edema formation are the same as has been described in adults (Starling forces), the unique features of fetal physiology, including characteristics of fetal microcirculation and immaturity of the developing organs, predispose to fetal edema formation. This chapter aims to provide some insight into the underlying pathophysiological mechanisms and should help to rationalize and categories the myriad conditions that may give rise to fetal hydrops. A better understanding of the various processes at work in the generation of hydrops provides more information for parental counseling and for more targeted antepartum/perinatal treatments. The importance of postmortem examination, including ancillary investigations, cannot be emphasized enough. Recent publications highlight the usefulness of immunohistochemical assessment of lymphatic vessels in idiopathic cases. It has been also shown that, when postmortem examinations are completed along with clinical assessments, the underlying causes can be found in very high percentage of the cases (>90 %). Even if the underlying forces to edema formation are the same as has been described in adults (Starling forces), the unique features of fetal physiology, including characteristics of fetal microcirculation and immaturity of the developing organs, predispose to fetal edema formation. This chapter aims to provide some insight into the underlying pathophysiological mechanisms and should help to rationalize and categories the myriad conditions that may give rise to fetal hydrops. A better understanding of the various processes at work in the generation of hydrops provides more information