Infrastructure networks like gas and power grids deliver critical services and need to be operated with a high degree of reliability. However, these networks are subject to growing amounts of uncertainty due to intermittent renewable generation sources, uncertainty in flexible demand resources and interdependence between networks. Further, analyzing the behavior of these networks is complicated because of the nonlinear nature of the equations describing the steady-state behavior. In recent years, we have developed novel techniques for "inner feasible" approximations of nonconvex sets that describe allowable operating points for infrastructure systems. We analyze the tightness of these characterizations, and describe applications of these techniques for robust optimization in infrastructure networks. This is the first general approach for robust nonconvex optimization problems with feasibility guarantees. This is joint work with Konstantin Turitysn, Hung Nguyen, Suhyoun Yu, Enrique Mallada and John Simpson-Porco.