Platelets are crucial for the occlusion of vascular injuries to arrest bleeding. Central to this hemostatic function is their ability to rapidly adhere to subendothelial matrix proteins (primary adhesion) and to co-aggregate, thus forming a thrombus that is later reinforced by the coagulation system [1,2]. Considerable progress has been made over the last two decades in the development of flow chambers and microcapillaries to monitor and study thrombus formation outside of the circulation. Comparisons with in vivo methods of thrombus formation using genetically modified mice have shown that many of the regulatory processes determining arterial thrombosis can well be assessed with these in vitro flow devices, based on the perfusion of whole blood over a platelet-activating surface at physiological shear rates. Hence, these devices have provided novel insights into the hemostatic function of platelets, particularly in combination with enhanced contrast (e.g. differential interference contrast, DIC) and/or fluorescence microscopy. Improvements in microscope technology, the availability of high-resolution cameras and the high capacity storage of digital images now make it possible to study in real time platelet aggregation under flow (and thrombus formation in vivo). Furthermore, new software packages facilitate the possibility of specific data extraction from these video images. Accordingly, an increasing number of laboratories are using custom-made or commercial flow chamber devices for (semi-routine) testing of impaired or increased platelet function under flow conditions. However, several groups around the world have developed their own standards and tools for flow-based assays, which complicate the comparison of outcomes and of outcome parameters.