In the context of a large program of prenatal echocardiographic detection of fetal heart disease, special attention was paid to the in utero detection and identification of fetal heart rate disorders. This article focuses on 18 cases of tachycardiac fetuses. It provides information on how to identify the nature of the detected tachycardia, the hemodynamic consequences of tachycardia, the particular characteristics of fetal atrial flutter, and the efficacy of the transmaternal treatment of fetal tachycardia. It also describes an original way to grade and score the degree of fetal heart failure that proved useful for the assessment and monitoring of in utero therapeutic procedures. The results presented stress the efficacy of digoxin for fetal atrial flutter as well as the usefulness of prenatal echocardiography in the assessment of anatomical, functional, and rhythmic conditions of the fetal heart.