The reaction mechanism of electron transfer in redox protein has become one of the most challenging problems in modern biochemistry, and has actually been the subject of intense study over the last few years. Although our present understanding is far from complete, we are moving with increasing speed to a stage where we can account for some of the most fundamental features. Redox potential differences between the prosthetic groups, solvent exposure and steric accessibility of the redox center at the protein surface, electrostatic interactions involving charged groups at or near the reaction sites, and others, are factors which have been demonstrated to strongly influence the rate constants of biochemical electron transfer reactions. Much of our present knowledge of these factors has been obtained from experiments carried out with flavins as reductants of a number of redox proteins containing heme irons, iron-sulfur centers, or even flavins, as prosthetic groups (cf. [Cusanovich et al., 1987; Tollin et al., 1986b; 1987]). In this article we focus on our most recent studies of flavin-protein redox interactions carried out with the heme-containing flavomolybdo-enzyme nitrate reductase, two c-type cytochromes, c552 and c550, and plastocyanins from spinach and a green alga. The utilization of flavin laser flash photolysis in the study of both oxidation and reduction of redox proteins is discussed.