A charged particle traveling through a transparent medium at a speed faster than the velocity of light in this medium will produce photons of light — Cherenkov photons. These photons can be used to measure the intensity of β
−
radiation. The main advantage of this method compared to LSC (Liquid Scintillation Counting) is the higher efficiency of light collection due to the volume ratio of the LS cocktail and the original solution and to the possibility to use the original solution as it is. The chapter discusses the possibility to use the External Source (ES) method.