Stimuli-responsive polymer interfaces, encompassing polymer particles, self-assemblies, layers, films, and gels, exhibit remarkable adaptability to environmental changes such as pH, temperature, and light. Their polymer interfaces undergo reversible structural and/or property changes in response to external stimuli, enabling a wide range of innovative applications in diverse fields. The ability to engineer such interfaces with molecular precision enables precise control over interfacial phenomena, paving the way for innovative applications in drug delivery, biosensing, cell cultures, chemical reactions, and environmental remediation. This chapter delves into the fundamental principles governing the design of stimuli-responsive polymer interfaces, such as particles, self-assemblies, layers, films, and hydrogels, explores cutting-edge synthetic methodologies including controlled/living polymerizations, supramolecular assembly, and crosslinking strategies, and delineates their physical and chemical behavior in response to external stimuli such as pH, temperature, light, and biomolecular interactions. Furthermore, this chapter highlights their applications in areas such as drug delivery, sensing, cell cultures, chemical reactions, and environmental remediation. The versatility and tunability of these smart polymer interfaces make them highly attractive for developing adaptive systems that can respond dynamically to changing conditions.