  We make a comprehensive study of the theory and phenomenology of a low energy
supersymmetric standard model originating from a string-inspired $E_6$ grand
unified gauge group. The Exceptional Supersymmetric Standard Model (ESSM)
considered here is based on the low energy SM gauge group together with an
extra $Z'$ corresponding to an extra $U(1)_{N}$ gauge symmetry under which
right--handed neutrinos have zero charge. The low energy matter content of the
ESSM corresponds to three 27 representations of the $E_6$ symmetry group, to
ensure anomaly cancellation, plus an additional pair of Higgs--like doublets as
required for high energy gauge coupling unification. The ESSM is therefore a
low energy alternative to the MSSM or NMSSM. The ESSM involves extra matter
beyond the MSSM contained in three $5+5^*$ representations of SU(5), plus three
SU(5) singlets which carry $U(1)_{N}$ charges, one of which develops a VEV,
providing the effective $\mu$ term for the Higgs doublets, as well as the
necessary exotic fermion masses. We explore the RG flow of the ESSM and examine
theoretical restrictions on the values of new Yukawa couplings caused by the
validity of perturbation theory up to the Grand Unification scale. We then
discuss electroweak symmetry breaking and Higgs phenomenology, and establish an
upper limit on the mass of the lightest Higgs particle which can be
significantly heavier than in the MSSM and NMSSM, in leading two--loop
approximation. We also discuss the phenomenology of the $Z'$ and the extra
matter, whose discovery will provide a smoking gun signal of the model.
