In situ gellable hydrogels represent a transformative class of biomaterials in tissue engineering, enabling minimally invasive delivery of cells, growth factors, and therapeutics directly into defect sites where they solidify to form scaffolds mimicking the extracellular matrix (ECM). This book chapter summarizes eight key studies on injectable hydrogel systems, including alginate-silica hybrids, hyaluronic acid-alginate blends, chitosan-gelatin nanocomposites, and click-cross-linked hyaluronan networks, primarily for bone regeneration. These formulations leverage ionic cross-linking, thermosensitive transitions, and covalent chemistries to achieve rapid gelation (within minutes) under physiological conditions, superior mechanical strength (up to 50-fold enhancement), tunable degradation, and sustained release of bioactives like BMP-2 and vancomycin. In vitro assessments demonstrate enhanced cell viability (>95%), proliferation, and osteogenic differentiation, while in vivo models (e.g., rat osteomyelitis, pig mandibular defects) confirm effective bone formation, infection control, and vascularization without inflammation. Challenges include optimizing release kinetics and scalability, but these systems outperform traditional grafts by promoting integration and reducing surgical risks. Future directions involve multifunctional hybrids for personalized therapies.