A reverse thermogelling polymer aqueous solution is a free-flowing sol at a low temperature and becomes a semisolid gel as the temperature increases. It is expected to be a very promising biomaterial as a minimally invasive injectable system for drug delivery and tissue engineering applications. The principles of materials design are (1) balancing the hydrophobicity and hydrophilicty of a polymer, (2) controlling the topology of a polymer, (3) matching the degradation kinetics of a polymer with a specific biomedical application, and (4) controlling the biocompatibility of the material with a drug as well as a host. This article covers recent progress of reverse theromogelling biodegradable polymers based on aliphatic polyesters, polyphosphazenes, poloxamer derivatives, polysaccharides, polypeptides, poly(propylene phosphate)s, polyorthoesters, polycarbonates, polycyanoacrylates, and poly(N-(2-hydroxyethyl) methacrylamide-lactate)s. The material characteristics, driving forces or mechanism for sol-gel transition, and their biomedical applications are summarized. In addition, the authors' perspectives on future reverse theromogelling materials design are suggested.