Double-network hydrogels (DN hydrogels), with high water content, high mechanical strength and excellent biocompatibility, have become innovative materials in biomedicine. DN hydrogels can be fabricated using two hydrophilic polymers with contrasting structures: rigid, brittle and highly corsslinked first network; soft, ductile and low-crosslinked second network. These two networks with complementary properties and crosslinking density provide the rigid, tough and ductile structure in DN hydrogels. Here, we will discuss the background, preparation, crosslinking structures of DN hydrogels, as well as their applications in tissue engineering.