The investigations described in this thesis started as part of the research program of the divisions of pediatric cardiology of the Universities of Arizona and California (San Diego). The investigations were part of an ongoing project designed by D.J. Sahn and L.M. Valdes-Cruz. This project was initiated to implement Doppler techniques in the daily practice of pediatric cardiology. New developments in medical technology made the Doppler techniques integrated into sophisticated two-dimensional echocardiographic equipment available to the clinician. In order to extend the use of the Doppler equipment beyond that of a sophisticated stethoscope and to enter the era of quantitative two-dimensional Doppler echocardiography it was felt that clinical validation of simple quantification approaches was necessary. As an initial approach animal studies were performed. Open chest dog models were created where simultaneously with the Doppler studies electromagnetic flow measurements and pressure gradients could be obtained. The results of both techniques were compared and a good correlation was obtained. Twodimensional Doppler echocardiographic quantifications were subsequently performed in patients with a variety of diseases during cardiac catheterization to validate the results in a clinical setting. The blood flow calculations over the mitral valve orifice and the development of the mitral valve flow method, including the mean to maximal index of the mitral valve, were performed by D. Fisher. Measurement of shunt size at atrial and ventricular level of the left-to- right type was validated by S. Horowitz. The measurement of extracardiac type left-to-right shunting such as in the persistent ductus arteriosus, of the tricuspid valve flow and a simplified approach to measurement of mitral valve flow will be described in this thesis. The tricuspid valve method and the simplified mitral valve methods are both studied in patients during cardiac catheterization for validation purposes and were subsequently used for calculation of flows in the human fetus during the second and third trimester of pregnancies. This thesis was completed by the study of the reproducibility of the previously described techniques in human subjects. This part of the investigation was performed at the Thoraxcenter in Rotterdam and supported by the Interuniversity Cardiology Institute of the Netherlands. This investigation was supervised by Professor Born and Professor Roeland!. H.Rijsterborgh performed the statistical analysis