  Spinor--Vector Duality (SVD) has been observed in worldsheet constructions of
heterotic--string compactifications. Recently, its realisation in the effective
field theory limit of string vacua in six and five dimensions has been
investigated. The SVD has been used to construct a string model that allows for
an extra family universal $U(1)$, with the standard $E_6$ embedding of its
charges, to remain unbroken down to low scales. Anomaly cancellation of the
extra $U(1)$ charges mandates the existence of additional matter states at the
extra $U(1)$ breaking scale, which affects precision measurements of Standard
Model parameters. I discuss the construction of non-supersymmetric sting vacua
and ``modular maps'' akin to the spacetime supersymmetry map. Such ``modular
maps'' provide a glimpse into the enormous symmetry structure underlying the
entire space of perturbative string vacua that is yet to be uncovered.
