Two types of black holes-stellar black holes and supermassive black holes-are known to exist in the universe. This leaves a large gap in the black hole mass distribution. In her Perspective, [Fabbiano][1] investigates whether ultraluminous x-ray sources may be the missing intermediate-mass black holes. She concludes that the evidence is inconclusive; most ultraluminous x-ray sources can be explained without resorting to the intermediate-mass black hole hypothesis, but some of the brightest sources may indeed be intermediate-mass black holes. [1]: http://www.sciencemag.org/cgi/content/full/307/5709/533