We describe how to use {\tt 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
