Photo-crosslinkable hydrogels have a wide application potential, especially in the fields of bioengineering and tissue engineering. Crosslinking of these hydrogels occurs under ultraviolet (UV) or visible light. When UV light is used, photoinitiators in polymer structures are activated and initiate crosslinking reactions by forming free radicals. This mechanism provides high structural integrity and mechanical strength in the formation of hydrogel networks. On the other hand, visible light offers advantages such as lower energy consumption and reduced negative effects on biological systems. In particular, visible light photo-crosslinking methods developed in recent years show higher biocompatibility to cells and tissues. As a mechanism, the activation of photosensitive photoinitiators and polymerization processes are similarly based on free radical production. These hydrogels are used in bioengineering applications; such as in tissue scaffolds, 3D bioprinting, and regenerative medicine. Adaptable structures of photo-crosslinkable materials are critical in terms of preserving the cellular structure and increasing cell viability. This chapter sheds light on current and future applications of photo-crosslinking processes using UV and visible light in bioengineering, highlighting the potential of these methods in tissue engineering and regenerative medicine.