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APDs (Avalanche Photodiodes) referred to in this Chapter differ by their construction and characteristics from those commonly used in long-distance optical communication. Common to both applications is the usage of an internal gain mechanism that functions by applying an adequate reverse voltage. In the optical communication industry one is mainly interested in small diameter devices to be coupled to optical fibres in near infrared domain. In nuclear physics they are used to convert light pulses, induced by particles and photons in scintillating crystals, into electronic signals. These emit at shorter wavelengths, moreover, it is advantageous to cover up to several cm2 of scintillator exit face with the sensing element to maximize the detector signal for low energies deposited in the scintillator. Therefore, progress in large area APDs in short-wavelength domain has been mainly driven by nuclear physics applications. A vast amount of research and development work invested by the joint CERN + Hamamatsu Photonics team resulted in a 5x5 mm2 device (S8664-55), which paved the way to larger area APDs. A notable feature of S8664-55 is its outstanding radiation hardness, so that by its application, the CMS-ECAL expects 10 years of failureless operation in a hostile radiation environment of the CERN-LHC. Let numbers illustrate the volume of APD usage: the barrel part of CMS-ECAL has 122400 pieces and the ALICE-PHOS 35840 of them. The meaning of the term High-Energy Gamma-Ray used in the title and meant in the rest of this chapter is related roughly to the maximum antiproton energy of 14 GeV from the HESR accumulator at the future FAIR facility in Darmstadt. The electromagnetic calorimeter (EMC) (Erni et al., 2008) of the PANDA detector, to be used in studies of hadron physics in antiproton-proton annihillations, will thus deal with photons in the energy range extending nearly from zero up to the maximum energy of the order of 10 GeV. Low-Energy Photon does not have its common meaning of a photon emitted from a radioactive source but rather is related to the practically achievable EMC low-energy detection threshold of several MeV. The results reported in Sect. 7 refer to a study intended to investigate detector resolution with 4 20 MeV photons, relevant for the latter energy range. Low-energy proton capture reactions are used to generate these gamma-rays. A reader interested in medical application of APDs, for example in Positron Emission Tomography (PET), which uses photons emitted in annihilation of positrons from radioactive Low-Energy Photon Detection with PWO-II Scintillators and Avalanche Photodiodes in Application to High-Energy Gamma-Ray Calorimetry