  The loop variable approach is a proposal for a gauge invariant generalization
of the sigma-model renormalization group method of obtaining equations of
motion in string theory. The basic guiding principle is space-time gauge
invariance rather than world-sheet properties. In essence it is a version of
Wilson's exact renormalization group equation for the world sheet theory. It
involves all the massive modes and is defined with a finite world-sheet cutoff,
which allows one to go off the mass-shell. On shell the tree amplitudes of
string theory are reproduced. The equations are gauge invariant off shell also.
This article is a self-contained discussion of the loop variable approach as
well as its connection with the renormalization group.
