Current ultrasonic blood flow velocity measurement systems are subject to a number of limitations, including limited frame rate, aliasing artifacts, and that only the velocity component along the ultrasound beam is estimated. This dissertation aims at solving some of these problems. The main part of the thesis considers a method for estimating the two-dimensional velocity vector within the image plane. This method, called synthetic aperture vector flow imaging, is first shortly reviewed. The main contribution of this work is partly an analysis of the method with respect to focusing effects, motion effects, and clutter rejection, and partly within in-vivo applications of the method, showing examples from various arteries and veins. Furthermore, two additional projects are described and tested. The first is an encoding method for simultaneously acquiring multiple lines for conventional velocity estimation. The method can be used for increasing the frame rate of color flow maps or alternatively for a new imaging modality entitled quadroplex imaging, featuring a color flow map and two independent spectrograms at a high frame rate. The second is an alternative method for ultrasonic vector velocity estimation. Two different velocity estimators were derived for finding both the axial and lateral velocity components through a multi-dimensional spectrum analysis. The work resulted in four journal papers and six conference papers, which are appended to the dissertation.