  We discuss the production, at high--energy e$^+$e$^-$ linear colliders, of
new heavy fermions predicted by extensions of the Standard Model. We analyze in
great details the various signals and the corresponding backgrounds for both
pair production and single production in association with ordinary fermions.
Concentrating on new leptons, we use a model detector for e$^+$e$^-$ collisions
at a center of mass energy of 500 GeV, to illustrate the discovery potential of
the Next Linear Colliders.
