Injectable hydrogels are adaptable drug delivery systems capable of forming localized depots that align with the anatomical and physiological constraints of administration sites. Their performance depends on both the injection environment and the properties of the therapeutic cargo. Applications span ocular, intra-articular, subcutaneous, intramuscular, tumoral, central nervous system, and mucosal delivery, where hydrogels address challenges of clearance, retention, and compatibility. Beyond bulk depots, particulate hydrogel formats such as microgels and nanogels improve syringeability, modularity, and integration with nanoparticle carriers. Functional versatility arises from stimuli responsiveness, including pH, enzymatic, thermal, redox, and light triggers, and from hybrid designs that integrate multiple cues for precision control. Loading strategies range from passive encapsulation to affinity binding and covalent conjugation, with release governed by diffusion, degradation, and stimuli-modulated kinetics. Translational progress depends on reproducible fabrication, scalable manufacturing, and device integration, while site-dependent constraints and regulatory hurdles remain significant challenges.