This chapter discusses the fluorescence energy transfer measurements of distances in rhodopsin and the purple membrane protein. Energy transfer measurements have provided information about the shape of the rhodopsin molecule. Rhodopsin in intact disk membranes can be specifically labeled with a wide variety of amines by using transglutaminase to catalyze a transamidation reaction. The rate of energy transfer can be markedly enhanced by translational diffusion occurring during the excited-state lifetime of the energy donor. The sensitivity of rapid-diffusion energy transfer to the closest approach distance makes it a choice method for ascertaining the distance between a chromophore and the surface of a protein or membrane. The location of the retinal chromophore of rhodopsin relative to the inside and outside surfaces of bovine retinal disk membranes has been determined by rapid-diffusion energy transfer measurements.