Our fundamental FRG flow equation (7.51) for the generating functional of the irreducible vertices and the equivalent hierarchy of integro-differential equations discussed in Sects. 7.3 and 7.4 are very complicated mathematical objects which usually cannot be solved exactly.1 It is therefore important to develop reliable approximation strategies for solving these equations. Roughly, two different types of strategies have been developed. The first is based on a suitable truncation of the hierarchy of integro-differential equations for the vertices. This vertex expansion approach was pioneered by Morris (1994) and has been extensively used in the condensed matter community.