  Spacetime non-commutativity appears in string theory. In this paper, the
non-commutativity in string theory is reviewed. At first we review that a
Dp-brane is equivalent to a configuration of infinitely many D($p-2$)-branes.
If we consider the worldvolume as that of the Dp-brane, coordinates of the
Dp-brane is commutative. On the other hand if we deal with the worldvolume as
that of the D($p-2$)-branes, since coordinates of many D-branes are promoted to
matrices the worldvolume theory is non-commutative one. Next we see that using
a point splitting reguralization gives a non-commutative D-brane, and a
non-commutative gauge field can be rewritten in terms of an ordinary gauge
field. The transformation is called the Seiberg-Witten map. And we introduce
second class constraints as boundary conditions of an open string. Since
Neumann and Dirichlet boundary conditions are mixed in the constraints when the
open string is coupled to a NS B field, the end points of the open string is
non-commutative.
