Many fishes are capable of extraordinary athletic feats, including short sprints at blistering speeds or migrations across entire oceans. This performance is all the more impressive when one considers that these fishes are breathing water, which contains ~30X less oxygen than the air that we breathe. One of our lab’s primary interests is in understanding how this is achieved. We have used computational fluid dynamics and quantitative flow visualization to thoroughly examine how water passes through the gills of several fish species. Our results suggest that the flow patterns around the gills have dramatic and previously unrecognized effects on oxygen uptake. Such results provide new perspectives on the respiratory physiology of fishes and perhaps other aquatic animals.