The Breit interaction is reviewed with applications to heavy atoms in mind. Generalizations of the Breit interaction which avoid expansion in powers of the electron velocities are discussed. Two-particle matrix elements of the Breit interaction and its generalizations are given in a form convenient for numerical applications. Expressions are derived for evaluating configuration-averaged atomic energy shifts for the Breit interaction and its generalizations. Numerical results for the energy shifts of atomic ground states are presented for selected atoms in the range Z=2 to Z=102; interpolated values of the energy shifts are given graphically for all atoms in the range considered. A breakdown of the interelectron contributions to the Breit energy shift is given for Ne and for K electrons in Hg. "Frozenorbital" calculations of Breit corrections to electron binding energies in Hg are given. The binding of K electrons in W, Hg, Pb, and Rn including the generalized Breit interaction with rearrangement are determined; when considered together with Lamb shift and correlation effects, these calculations reduce the discrepancy between theoretical and experimental K binding energies to about 0.1 Ry.