This review explores various approaches to introduce bioactivity into poly(ethylene glycol) (PEG) hydrogels, which are excellent scaffolding materials for tissue engineering due to their ability to provide a highly swollen three-dimensional (3D) environment similar to soft tissues. While PEG hydrogels have advantages over natural hydrogels, such as adjustable mechanical properties and easy control of scaffold architecture, they lack the bioactivity necessary for supporting cell adhesion and tissue formation due to their bio-inert nature. The extracellular matrix (ECM) has been a valuable model for designing bioactive scaffolds, and ECM-mimetic modifications of PEG hydrogels have emerged as a key strategy to modulate specific cellular responses. Various strategies have been developed to incorporate key ECM biofunctions, such as specific cell adhesion, proteolytic degradation, and signal molecule binding, into PEG hydrogels. This review covers recent progress in material designs and fabrication approaches leading to the development of bioactive PEG hydrogels as tissue engineering scaffolds, including the use of ECM-derived bioactive molecules, post-grafting, free radical polymerization, thiol-acrylate photopolymerization, Click chemistry, enzymatic reactions, and photoregulation of hydrogel bioactivity. Despite these advancements, challenges remain in achieving precise spatial and temporal control of scaffold properties and maintaining long-term biocompatibility.This review explores various approaches to introduce bioactivity into poly(ethylene glycol) (PEG) hydrogels, which are excellent scaffolding materials for tissue engineering due to their ability to provide a highly swollen three-dimensional (3D) environment similar to soft tissues. While PEG hydrogels have advantages over natural hydrogels, such as adjustable mechanical properties and easy control of scaffold architecture, they lack the bioactivity necessary for supporting cell adhesion and tissue formation due to their bio-inert nature. The extracellular matrix (ECM) has been a valuable model for designing bioactive scaffolds, and ECM-mimetic modifications of PEG hydrogels have emerged as a key strategy to modulate specific cellular responses. Various strategies have been developed to incorporate key ECM biofunctions, such as specific cell adhesion, proteolytic degradation, and signal molecule binding, into PEG hydrogels. This review covers recent progress in material designs and fabrication approaches leading to the development of bioactive PEG hydrogels as tissue engineering scaffolds, including the use of ECM-derived bioactive molecules, post-grafting, free radical polymerization, thiol-acrylate photopolymerization, Click chemistry, enzymatic reactions, and photoregulation of hydrogel bioactivity. Despite these advancements, challenges remain in achieving precise spatial and temporal control of scaffold properties and maintaining long-term biocompatibility.