Stimuli-responsive fluids and gels are typically capable of changing their properties—primarily viscoelasticity—with field effects from light, temperature, magnetic behavior, and biological response. This means that the fluids have associative or dissociative behavior that can be triggered or controlled to result in gelation or phase behavior and equilibration. This review will focus on a number of polymeric and blend systems that contain functional groups capable of responding to stimuli. The key is in controlling the mechanism or the degree to which these materials respond to enable truly smart applications.