  The strong coupling dynamics of string theories in dimension $d\geq 4$ are
studied. It is argued, among other things, that eleven-dimensional supergravity
arises as a low energy limit of the ten-dimensional Type IIA superstring, and
that a recently conjectured duality between the heterotic string and Type IIA
superstrings controls the strong coupling dynamics of the heterotic string in
five, six, and seven dimensions and implies $S$ duality for both heterotic and
Type II strings.
