The interaction of electromagnetic radiation and charged particles with crystals may involve ionization, i.e., the production of excess current carriers. Experiments with single crystals of silicon have confirmed theoretical predictions on the effect of an externally applied electric field on the process of photoionization. A study of photoionization in the inner portions of the fundamental optical absorption bands of germanium and silicon has shown that at sufficiently high photon energies the quantum yield rises to values considerably greater than unity. For photons of energies many times greater than the width of the forbidden band, the quantum yield is proportional to the energy of the photon. In ionization due to fast charged particles, the energy lost per electron-hole pair produced is independent of the particle energy.