The echogenicity of blood is dependent of the back scattering of the ultra-sound beam by the flowing blood-stream. This black scattering is, according to the Rayleigh theory, proportional to the fourth power of the frequency and to the size of the particles. So, for the frequencies in clinical use, the size of the particles is essential. Micro bubbles and aggregates of red cells can be echogenic. Micro bubbles are scare. Echoes are mainly generated by blood cells aggregated from a given size for each wave length. For instance, 225 for 7.5 MHz frequency. High degree hematocrits and big molecules induce aggregation, but the main factor is the flow speed or more exactly the shear rate, i.e. 8/3 of speed/vessel radius. In clinical practice, blood becomes echogenic if flow slows and if the vessel radius increases. This happens for instance for venous or even arterial aneurysms, for dilated and dyskinetic cardiac cavities, an above all, in veins when flow slows. Echogenicity appears proximal to an organic or hemodynamic obstacle and is reversible when flow is restored. The technical conditions are important, too. Blood becomes more echogenic when the scan benears, the frequency increases and the resolution of the device goes higher. It can be expected that hemodynamic and even rheologic information will be obtainable in big vessels with computerized techniques quantifying blood echogenicity. Blood clots will be echogenic under the same conditions: red blood cells aggregated non hemolyzed. Their echogenicity appears more dependent of their structure than of the chronology. Better technical conditions will increase the clot echogenicity, too. Therapeutic and prognostic conclusions can be expected by better evaluation of prethrombotic stages and of structure of blood clots.