|Zebrafish embryos are transparent, permitting an unprecedented level of direct observation during embryonic development. Relating to our research interests, this transparency allows us to easily observe circulatory function in live embryos (bottom left). We take further advantage of this transparency through the generation of transgenic zebrafish in which endothelial cells express green or red fluorescent protein. Coupled with confocal or 2-photon microscopy, we can directly image blood vessels as they grow in a live embryo through time-lapse (middle panel), or obtain high-content imaging for 3D rendering (bottom right). The external development of transparent zebrafish embryos also facilitates their use in moderate scale small molecule screens coupled with high-throughput imaging - an effort we have recently begun to apply.