In the past two decades, physical polymeric hydrogels have been extensively explored for biomedical applications, such as drug delivery and tissue engineering. These hydrogels exhibit a sol-gel phase transition in response to Sol external stimuli, such as pH, temperature, glucose, electric field, magnetic field, ionic strength, or their combination. Hydrogel precursors can be mixed with bioactive molecules or cells and then simply injected into the body at specific sites. Substantial progress has been made in the development of novel hydrogels and applications. In this Perspective, we report recent progress in physically cross-linked,hydrogels responding to pH and/or temperature and their pharmaceutical and tissue engineering applications. The outlook for the future, including remaining challenges of injectable hydrogels, is also discussed.