Orbifolds of M-Theory and String Theory

It is shown that many of the conjectured dualities involving orbifold compactification of M-theory follow from the known dualities involving M-theory and string theory in ten dimensions, and the ansatz that orbifolding procedure commutes with the duality transformation. This ansatz also leads to a new duality conjecture, namely that M-theory compactified on $T^8/Z_2$ is dual to type I string theory on $T^7$. In this case the `twisted sector states' in M-theory live on sixteen membranes transverse to the internal manifold.