String theory dynamics in various dimensions

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 and six dimensions and implies S duality for both heterotic and Type II strings