Abstract
The effective interaction between interfaces or membranes is renormalized by thermally excited shape fluctuations of these surfaces. For a large class of interactions, this renormalization leads to a complex phase diagram which is governed by an unusual renormalization-group flow. This flow exhibits a line of renormalization-group fixed points and leads to essential singularities and nonuniversal critical exponents; it must, however, be distinguished from the well-known Kosterlitz-Thouless flow since it has a parabolic character.