This review discusses the synthesis and properties of injectable hydrogels for tissue filling. Hydrogels, known for their minimal adverse effects, fast recovery, and good results, have gained attention due to their injectability, which can be achieved through chemical, physical, or biological crosslinking. Chemical covalent crosslinking, such as dynamic covalent bonds (e.g., Michael addition, Diels-Alder, Schiff base reactions), radical polymerization, and delayed gelatinization, allows for reversible sol-gel transitions and enhanced stability. Physical crosslinking, including hydrogen bonding, hydrophobic interactions, host-guest interactions, and ionic interactions, provides reversible sol-gel transitions without the need for additional crosslinking agents. Biological crosslinking, primarily through enzymatic reactions, offers high efficiency, selectivity, and excellent biosafety. The composition of injectable hydrogels includes both natural polymers (e.g., hyaluronic acid, silk fibroin, collagen) and synthetic polymers (e.g., polyvinyl alcohol, polyethylene glycol, polylactic acid). These materials are chosen for their unique properties, such as biodegradability, mechanical strength, and biological functions. The mechanical strength, degradation rate, and biological function of injectable hydrogels are crucial for their success in tissue filling applications. HA-based, SF-based, and collagen-based hydrogels are highlighted for their potential in soft tissue filling, with HA being the most commercially popular due to its viscoelasticity and biodegradability. However, challenges such as poor mechanical strength and stability can be addressed by crosslinking or adding other biomaterials. Overall, injectable hydrogels offer a promising approach for tissue augmentation and regeneration, with ongoing research focusing on optimizing their therapeutic potential and addressing existing challenges.This review discusses the synthesis and properties of injectable hydrogels for tissue filling. Hydrogels, known for their minimal adverse effects, fast recovery, and good results, have gained attention due to their injectability, which can be achieved through chemical, physical, or biological crosslinking. Chemical covalent crosslinking, such as dynamic covalent bonds (e.g., Michael addition, Diels-Alder, Schiff base reactions), radical polymerization, and delayed gelatinization, allows for reversible sol-gel transitions and enhanced stability. Physical crosslinking, including hydrogen bonding, hydrophobic interactions, host-guest interactions, and ionic interactions, provides reversible sol-gel transitions without the need for additional crosslinking agents. Biological crosslinking, primarily through enzymatic reactions, offers high efficiency, selectivity, and excellent biosafety. The composition of injectable hydrogels includes both natural polymers (e.g., hyaluronic acid, silk fibroin, collagen) and synthetic polymers (e.g., polyvinyl alcohol, polyethylene glycol, polylactic acid). These materials are chosen for their unique properties, such as biodegradability, mechanical strength, and biological functions. The mechanical strength, degradation rate, and biological function of injectable hydrogels are crucial for their success in tissue filling applications. HA-based, SF-based, and collagen-based hydrogels are highlighted for their potential in soft tissue filling, with HA being the most commercially popular due to its viscoelasticity and biodegradability. However, challenges such as poor mechanical strength and stability can be addressed by crosslinking or adding other biomaterials. Overall, injectable hydrogels offer a promising approach for tissue augmentation and regeneration, with ongoing research focusing on optimizing their therapeutic potential and addressing existing challenges.