In situ gellable hydrogels represent a transformative advancement in biomedical applications due to their ability to undergo sol-gel transition under physiological conditions. These materials offer significant benefits for targeted drug delivery, tissue engineering, wound healing, and regenerative medicine, owing to their high biocompatibility, tunable mechanical properties, and responsiveness to environmental stimuli such as pH, temperature, and ionic strength. Recent trends focus on the development of smart, multifunctional hydrogels that incorporate self-healing, antimicrobial, and stimuli-responsive behaviors. Innovations in fabrication techniques, including 3D printing and electrospinning, enable the creation of complex structures that mimic the native extracellular matrix, supporting cell growth and personalized therapeutic strategies. This book chapter comprehensively reviews the classification, properties, and emerging applications of in situ hydrogels, highlighting their potential to revolutionize biomedical fields through enhanced efficacy, minimized invasiveness, and tailored therapeutic outcomes.