We show that in certain superstring compactifications, gauge theories on
noncommutative tori will naturally appear as D-brane world-volume theories.
This gives strong evidence that they are well-defined quantum theories. It also
gives a physical derivation of the identification proposed by Connes, Douglas
and Schwarz of Matrix theory compactification on the noncommutative torus with
M theory compactification with constant background three-form tensor field.Comment: harvmac, 5 pp. Minor error fixe