We describe ZFITTER, a Fortran program based on a semi-analytical approach to fermion pair production in e+e- annihilation at a wide range of centre-of-mass energies, including the PETRA, TRISTAN, LEP1/SLC, and LEP2 energies. A flexible treatment of complete O(alpha) QED corrections and of some higher order contributions is made possible with three calculational chains containing different realistic sets of restrictions in the photon phase space. Numerical integrations are at most one-dimensional. Complete O(alpha) weak loop corrections supplemented by selected higher-order terms may be included. The program calculates Delta r, the Z width, differential cross-sections, total cross-sections, integrated forward-backward asymmetries, left-right asymmetries, and for tau pair production also final-state polarization effects. Various interfaces allow fits to be performed with different sets of free parameters.