We describe how to use ZFITTER, a program based on a semi-analytical approach to fermion pair production in e+e- annihilation and Bhabha scattering. A flexible treatment of complete ${cal O}(alpha)$ QED corrections, also including higher orders, allows for three calculational {bf chains} with different realistic sets of restrictions in the photon phase space. {tt ZFITTER} consists of several {bf branches} with varying assumptions on the underlying hard scattering process. One includes complete ${cal O}(alpha)$ weak loop corrections with a resummation of leading higher-order terms. Alternatively, an ansatz inspired from S-matrix theory, or several model-independent effective Born cross sections may be convoluted. The program calculates cross sections, forward-backward asymmetries, and for $tau$~pair production also the final-state polarization. Various {bf interfaces} allow fits to be performed with different sets of free parameters.
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