  In this article we present a comprehensive account of the operator
formulation of the Green-Schwarz superstring in the semi-light-cone (SLC)
gauge, where the worldsheet conformal invariance is preserved. Starting from
the basic action, we systematically study the symmetry structure of the theory
in the SLC gauge both in the Lagrangian and the phase space formulations. After
quantizing the theory in the latter formulation we construct the quantum
Virasoro and the super-Poincare generators and clarify the closure properties
of these symmetry algebras. Then by making full use of this knowledge we will
be able to construct the BRST-invariant vertex operators which describe the
emission and the absorption of the massless quanta and show that they form the
appropriate representation of the quantum symmetry algebras. Furthermore, we
will construct an exact quantum similarity transformation which connects the
SLC gauge and the familiar light-cone (LC) gauge. As an application
BRST-invariant DDF operators in the SLC gauge are obtained starting from the
corresponding physical oscillators in the LC gauge.
