### abstract

- Non perturbative effects in string theory are important. To someone familiar with weakly coupled quantum field theories that may sound a little strange. After all, non- perturbative effects are much smaller than perturbative effects in such theories. Typi- cally, in a weakly coupled field theory, non- perturbative effects have strength e -1/g2 com- pared to perturbative quantities that have strength g2, where g is the coupling param- eter of the theory. For example if g2 = .3 the ratio of these numbers is about 1 : 100. The only case in which non-perturbative quan- tities are important is when their value in per- turbation theory vanishes. This is the situ- ation in superstring theory for a number of important quantities, eg, the size of super- symmetry breaking (1). The smallness of non- perturbative effects is actually an advantage in string theory because it may explain the small ratio of the electroweak scale and Planck scale.