The kinematics of electron-positron pair production and annihilation, i.e., the determination and transformation of the momenta and energies of particles and photons upon the transition from an arbitrary reference frame to the center-of-mass frame of the particles and back, is analyzed in detail. It is found that the magnitudes of the particle momenta in certain directions in pair production may be ambiguous. An interpretation of this ambiguity and a way of circumventing it are given. Invariant quantities and the most convenient variables for calculating various integrals are found. Then the differential and total cross sections are given and the mean frequencies and dispersions of the frequencies of photons produced during annihilation are calculated. © 1999 Kluwer Academic/Plenum Publishers.