Stimulus-responsive hydrogels have become essential materials in biomedical and industrial applications due to their ability to respond to external stimuli such as temperature, pH, light, and shear stress. This review discusses the theory, modern advances, and applications of these hydrogels, highlighting their unique properties and the mechanisms that enable their responsive behavior. Hydrogels are three-dimensional networks of polymeric chains that can swell in water, offering high biocompatibility and soft physical properties similar to living tissues. Stimulus-responsive hydrogels allow for precise control over material properties, enabling breakthroughs in healthcare, including more effective treatments for diseases and improved tissue engineering and wound healing. The review covers various response modalities, including temperature, pH, chemical, light, electro, and shear-sensitive hydrogels. It discusses the theoretical analysis of hydrogel properties and the mechanisms used to create these responses, highlighting both pioneering and recent work in these fields. The review also reviews current and proposed applications of these hydrogels in medicine and industry. Hydrogels are characterized by their physical structure, which includes crosslinked polymeric chains that can take various forms such as covalent chemical links, ionic bonds, and hydrogen bonds. The equilibrium swelling theory and network characteristics are discussed, including the Flory–Rehner theory for nonionic hydrogels and the additional term for ionic hydrogels. The mesh size parameter, which quantifies hydrogel porosity, is also discussed, along with the factors that influence it. Dual responsive hydrogels, composed of interpenetrating polymer networks (IPNs), are capable of responding to multiple stimuli due to the unique environmental responsiveness of each polymer network. The review discusses the modeling of IPN swelling behavior, including the assumptions of homogeneous behavior, IPN interactions, and independent network behavior. The mechanical behavior of hydrogels is crucial for their application in biomedical contexts, as it influences their ability to withstand and respond to external forces. The viscoelastic properties of hydrogels are influenced by their crosslinking density and the presence of solvent, which can significantly affect their performance. Solute transport in hydrogels is crucial for their use as drug delivery systems, as it determines the rate and extent of drug release. The review discusses the factors that influence solute transport, including mesh size, pH, and temperature, and highlights the development of pH-responsive hydrogels that exhibit near zero-order release. pH-responsive hydrogels are particularly useful for targeted drug delivery, as they can respond to changes in pH in different parts of the body. The review discusses the composition and swelling behavior of pH-responsive hydrogels, as well as their applications in controlled drug delivery, including oral delivery of therapeutic proteins and other drugs. The review also highlights the importance of factors such as ionic strength, solute characteristics, and network properties in determining the overall swelling behavior of ionic hydrogels.Stimulus-responsive hydrogels have become essential materials in biomedical and industrial applications due to their ability to respond to external stimuli such as temperature, pH, light, and shear stress. This review discusses the theory, modern advances, and applications of these hydrogels, highlighting their unique properties and the mechanisms that enable their responsive behavior. Hydrogels are three-dimensional networks of polymeric chains that can swell in water, offering high biocompatibility and soft physical properties similar to living tissues. Stimulus-responsive hydrogels allow for precise control over material properties, enabling breakthroughs in healthcare, including more effective treatments for diseases and improved tissue engineering and wound healing. The review covers various response modalities, including temperature, pH, chemical, light, electro, and shear-sensitive hydrogels. It discusses the theoretical analysis of hydrogel properties and the mechanisms used to create these responses, highlighting both pioneering and recent work in these fields. The review also reviews current and proposed applications of these hydrogels in medicine and industry. Hydrogels are characterized by their physical structure, which includes crosslinked polymeric chains that can take various forms such as covalent chemical links, ionic bonds, and hydrogen bonds. The equilibrium swelling theory and network characteristics are discussed, including the Flory–Rehner theory for nonionic hydrogels and the additional term for ionic hydrogels. The mesh size parameter, which quantifies hydrogel porosity, is also discussed, along with the factors that influence it. Dual responsive hydrogels, composed of interpenetrating polymer networks (IPNs), are capable of responding to multiple stimuli due to the unique environmental responsiveness of each polymer network. The review discusses the modeling of IPN swelling behavior, including the assumptions of homogeneous behavior, IPN interactions, and independent network behavior. The mechanical behavior of hydrogels is crucial for their application in biomedical contexts, as it influences their ability to withstand and respond to external forces. The viscoelastic properties of hydrogels are influenced by their crosslinking density and the presence of solvent, which can significantly affect their performance. Solute transport in hydrogels is crucial for their use as drug delivery systems, as it determines the rate and extent of drug release. The review discusses the factors that influence solute transport, including mesh size, pH, and temperature, and highlights the development of pH-responsive hydrogels that exhibit near zero-order release. pH-responsive hydrogels are particularly useful for targeted drug delivery, as they can respond to changes in pH in different parts of the body. The review discusses the composition and swelling behavior of pH-responsive hydrogels, as well as their applications in controlled drug delivery, including oral delivery of therapeutic proteins and other drugs. The review also highlights the importance of factors such as ionic strength, solute characteristics, and network properties in determining the overall swelling behavior of ionic hydrogels.