Space-time uncertainty and noncommutativity in string theory

We analyze the nature of space-time nonlocality in string theory. After giv ing a brief overview on the conjecture of the space-time uncertainty princi ple, a (semi-classical) reformulation of string quantum mechanics, in which the dynamics is represented by the noncommutativity between temporal and s patial coordinates, is outlined. The formalism is then compared to the spac e-time noncommutative field theories associated with nonzero electric B-fie lds.