Cherenkov radiation emission occurs in all dielectric media (such as water and biological tissue) when charged particles move with phase velocity greater than the speed of light in that medium.1 This Cherenkov effect produces a broad spectrum of light emission from UV down to near-infrared, with a spectrum described by the Frank-Tamm formula, varying as the inverse square of wavelength.2 Recent work has shown, in phantoms, small animals,3 and clinical patients,4 that Cherenkov emission is generated by and emitters in diagnostic (18-FDG) and nuclear medicine (131-I) applications. However, to date, Cherenkov radiation emission has never been imaged in vivo during external beam radiation therapy (EBRT), where there is considerable signal throughout the treated regions of tissue. In this letter, we present the first linearity test between surface dose and Cherenkov emission in a phantom, followed by the first video sequence of Cherenkov emission in vivo. This imaging is here named Cherenkoscopy, for the parallel between fluoroscopy but with surface imaging of Cherenkov signal. The imaging was performed while a dog was undergoing planned EBRT for treatment of a spontaneous oral tumor.