Stimuli-responsive polymers or so-called “smart polymers” are macromolecules that are sensitive to certain triggers from the external environment, including temperature, light, electrical or magnetic fields, and chemicals. The activated polymers produce observable or detectable micro- or nanoscale changes, such as morphology, molecular bond rearrangement/cleavage, and molecular motion, which can induce changes in their macroscopic properties such as color, shape, and functionality. Due to the versatile selection of backbone and functional groups, stimuli-responsive polymers can be tailored to have a variety of specific mechanical, chemical, electrical, optical, biological, or other properties and can be engineered into different forms, including bulk, thin film, micro/nanoparticles, and composites. Over the years, many multidisciplinary efforts have been conducted and reported optimizing the functionality of stimuli-responsive polymers and exploring new and innovative applications. However, as shown below, original and exciting research in emerging sectors continues to drive the evolution of and interest in this class of polymer.