Nathan Seiberg and Edward Witten explore the appearance of noncommutative geometry in string theory with a nonzero $B$-field. They identify a limit where the entire string dynamics is described by a minimally coupled (supersymmetric) gauge theory on a noncommutative space and discuss corrections away from this limit. Their analysis leads to an equivalence between ordinary gauge fields and noncommutative gauge fields, realized by a change of variables that can be explicitly described. This change of variables is verified by comparing the ordinary Dirac-Born-Infeld theory with its noncommutative counterpart. The paper also discusses the noncommutative gauge theory on a torus, its $T$-duality, and Morita equivalence, as well as the $D0/D4$ system, its relation to $M$-theory in DLCQ, and a possible noncommutative version of the six-dimensional $(2,0)$ theory. The authors further examine the behavior of instantons at nonzero $B$ by quantizing the D0-D4 system and explore the noncommutative Yang-Mills theory on a torus, analyzing the action of $T$-duality. They conclude by reexamining the relation of noncommutative Yang-Mills theory to DLCQ quantization of $M$-theory and exploring the possible noncommutative version of the $(2,0)$ theory in six dimensions.Nathan Seiberg and Edward Witten explore the appearance of noncommutative geometry in string theory with a nonzero $B$-field. They identify a limit where the entire string dynamics is described by a minimally coupled (supersymmetric) gauge theory on a noncommutative space and discuss corrections away from this limit. Their analysis leads to an equivalence between ordinary gauge fields and noncommutative gauge fields, realized by a change of variables that can be explicitly described. This change of variables is verified by comparing the ordinary Dirac-Born-Infeld theory with its noncommutative counterpart. The paper also discusses the noncommutative gauge theory on a torus, its $T$-duality, and Morita equivalence, as well as the $D0/D4$ system, its relation to $M$-theory in DLCQ, and a possible noncommutative version of the six-dimensional $(2,0)$ theory. The authors further examine the behavior of instantons at nonzero $B$ by quantizing the D0-D4 system and explore the noncommutative Yang-Mills theory on a torus, analyzing the action of $T$-duality. They conclude by reexamining the relation of noncommutative Yang-Mills theory to DLCQ quantization of $M$-theory and exploring the possible noncommutative version of the $(2,0)$ theory in six dimensions.