Inspired by the responsiveness of natural systems to their surrounding environments, researchers have sought to understand these biological processes and to develop functional stimuli-responsive polymeric systems for a wide range of applications such as drug delivery, imaging, and regenerative medicine. Both synthetic polymers and biopolymers have been studied and incorporated into assemblies of different morphologies as well as hydrogels with diverse shapes and dimensions. This special issue highlights recent research advances in this area, as well as exciting challenges to be tackled in the upcoming years.