The discovery of emission and absorption of γ rays free of thermal Doppler broadening and the effects of recoil by R.L. MOSSBAUER [3.1] brought to experimental physics a tool that has been used to detect fractional changes of electromagnetic energy smaller than can be detected by any other technique. One of the earliest and most striking applications was the confirmation of the effect of gravity on photon energy as predicted by A. Einstein. The general background and some details of the most thorough of such studies will be described herein.