This review highlights recent developments in the field of stimuli-responsive hydrogels, focusing primarily on thin films, with a thickness range between 100 nm to 10 μm. The theory and dynamics of hydrogel swelling is reviewed, followed by specific applications. Gels are classified based on the active stimulus—mechanical, chemical, pH, heat, and light—and fabrication methods, design constraints, and novel stimuli-responses are discussed. Often, these materials display large physiochemical reactions to a relatively small stimulus. Noteworthy materials larger than 10 μm, but with response times on the order of seconds to minutes are also discussed. Hydrogels have the potential to advance the fields of medicine and polymer science as useful substrates for “smart” devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1084–1099