<p>Hydrogels are crosslinked polymer networks with a large amount of water; hydrogels can be found naturally, such as collagen and gelatin, or can be made synthetically. The latter are nowadays more in demand due to their higher water absorption capacity and long service life; their tunable properties and versatile fabrication methods have been exploited in a variety of engineering applications, including sensing technologies and drug screening. Because of their similar characteristics to soft biological tissues, hydrogels have also been the focus of research in the biomaterials community and have been studied for their use in biomedical applications such as tissue engineering and regenerative medicine. This Collection on hydrogels highlights some exciting hydrogel developments and applications.</p>