The Strength of Nonperturbative Effects in String Theory

We argue that the leading weak coupling nonperturbative effects in closed string theories should be of order exp(— C/κ) where κ 2 is the closed string coupling constant. This is the case in the exactly soluble matrix models. These effects are in principle much larger than the exp(—C/κ 2 ) effects typical of the low energy field theory. We argue that this behavior should be generic in string theory because string perturbation theory generically behaves like (2g)! at genus g.