X-ray reverberation has emerged as a powerful tool to probe black hole accretion at the scales of the event horizon. It relies on measuring light travel time delays between direct emission and its reflection off the surrounding dense material. These reverberation lags have been seen in many bright AGN, and they all imply very compact coronae. Combining these measurements with coronal temperatures with NuSTAR, we are learning new details about the illusive corona and its properties. I will discuss recent reverberation results and their implications on our understanding of the inner regions of accretion disks around black holes.