Aqueous polymer solutions that are transformed into gels by changes in environmental conditions, such as temperature and pH, thus resulting in in situ hydrogel formation, have recently attracted the attention of many investigators for scientific interest and for practical biomedical or pharmaceutical applications. When the hydrogel is formed under physiological conditions and maintains its integrity for a desired period of time, the process may provide various advantages over conventional hydrogels. Because of the simplicity of pharmaceutical formulation by solution mixing, biocompatibility with biological systems, and convenient administration, the pharmaceutical and biomedical uses of the water-based sol–gel transition include solubilization of low-molecular-weight hydrophobic drugs, controlled release, labile biomacromolecule delivery, such as proteins and genes, cell immobilization, and tissue engineering. When the formed gel is proven to be biocompatible and biodegradable, producing non-toxic degradation products, it will provide further benefits for in vivo applications where degradation is desired. It is timely to summarize the polymeric systems that undergo sol–gel transitions, particularly due to temperature, with emphasis on the underlying transition mechanisms and potential delivery aspects. This review stresses the polymeric systems of natural or modified natural polymers, N-isopropylacrylamide copolymers, poly(ethylene oxide)/poly(propylene oxide) block copolymers, and poly(ethylene glycol)/poly(d,l-lactide-co-glycolide) block copolymers.