Excited fermions at e+e- and eP colliders

We analyse the discovery potentials of future colliders with respect to excited fermions which are expected in models of fermionic substructure. After discussing the possibilites offered by LEP, we will consider in detail the Next Linear Collider in its e+e-, eγ and γγ modes [where the high-energy photon beams are obtained through back-scattering of laser light]. In addition, since eP colliders are well adapted to the search of the first generation of excited leptons, we include a study of their possible manifestation at HERA and LEP/LHC. We give complete and compact formulae for decay widths, production cross sections and angular distributions. Furthermore, we analyse in some detail the polarisation of the produced excited fermions which allows for an easy reconstruction of the correlations between the initial state and the decay products of the heavy fermions. (orig.)