Photoconductivity originates from the creation of mobile charge carriers by absorption of radiation of proper energy and their transport up to the electrode. We use the word “proper” and not “energy higher than the band gap” because several semiconductors show photoresponse for energy lower than the band-gap value due to impurities present in the photoconductors. Electronic transitions and the process of creation of mobile charge carriers depend upon the band structure of photoconductors, while their transport depends upon other factors, including the scattering process, mobilities, and recombination mechanisms. In this chapter we will deal only with band structure properties and electronic transitions that generate free charge carriers, which are responsible for the photoconduction process.