Electron transfer in proteins is discussed and compared to electron transfer in bridged donor–acceptor systems. Results from extensive ab initio calculations for the latter type of system are summarized. The electronic factor Δ is often nonexponential as a function of number of bonds in the bridge. Δ may even change sign as the number of bonds in the bridge is increased in a repetitive way. Correlation effects, including the “broken symmetry” correction, are studied and are found to be important in some cases. The variational problem is formulated as an interaction between a donor and an acceptor via “pathways” through the protein. © 1993 John Wiley & Sons, Inc.