  We extend earlier ideas about the appearance of noncommutative geometry in
string theory with a nonzero B-field. We identify a limit in which the entire
string dynamics is described by a minimally coupled (supersymmetric) gauge
theory on a noncommutative space, and discuss the corrections away from this
limit. Our analysis leads us to an equivalence between ordinary gauge fields
and noncommutative gauge fields, which is realized by a change of variables
that can be described explicitly. This change of variables is checked by
comparing the ordinary Dirac-Born-Infeld theory with its noncommutative
counterpart. We obtain a new perspective on noncommutative gauge theory on a
torus, its T-duality, and Morita equivalence. We also discuss the D0/D4 system,
the relation to M-theory in DLCQ, and a possible noncommutative version of the
six-dimensional (2,0) theory.
