  I discuss in detail the construction of realistic superstring standard--like
models in the four dimensional free fermionic formulation. The analysis results
in a restricted class of models with unique characteristics: (i) Three and only
three generations of chiral fermions with their superpartners and the correct
Standard Model quantum numbers. (ii) Proton decay from dimension four and
dimension five operators is suppressed due to gauged $U(1)$ symmetries. (iii)
There exist Higgs doublets from two distinct sectors, which can generate
realistic symmetry breaking. (iv) These models explain the top--bottom mass
hierarchy. At the trilinear level of the superpotential only the top quark gets
a non vanishing mass term. The bottom quark and the lighter quarks and leptons
get their mass terms from non renormalizable terms. This result is correlated
with the requirement of a supersymmetric vacuum at the Planck scale. (v) The
models predict the existence of small hidden gauge groups, like $SU(3)$, with
matter spectrum in vector representations.
