We summarize a framework for robustness analysis of nonlinear systems . The notion of model uncertainty is based on the closeness of input-output trajectories, and is not tied to a particular uncertainty representation, such as additive, parametric, structured, etc. The basic viewpoint is to regard systems as operators on signal spaces. We outline a global theory and a local approach (over bounded signal sets) which applies to systems with potential for finite escape. Several examples are presented, as well as a generalization of the framework to the case where uncertainty occurs in several subsystems in an arbitrary interconnection.