This article reviews the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels, which are highly swollen polymeric networks, are attractive due to their ability to encapsulate cells and bioactive molecules, efficient mass transfer, and biocompatibility. Various polymers, including natural, synthetic, and hybrid polymers, have been used to create hydrogels through chemical or physical crosslinking. Synthetic hydrogels, in particular, have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mechanical properties, mimicking the extracellular matrix (ECM) for cell growth and tissue formation. The article discusses strategies for designing synthetic hydrogels with ECM-mimetic properties, such as cell adhesion, proteolytic degradation, and growth factor binding. It also highlights the importance of controlling the network structure of hydrogels to ensure proper degradation, diffusion of bioactive molecules, and cell migration. The article concludes with a discussion on the challenges and future perspectives in the design and synthesis of bioactive or biomimetic hydrogels for tissue engineering applications.This article reviews the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels, which are highly swollen polymeric networks, are attractive due to their ability to encapsulate cells and bioactive molecules, efficient mass transfer, and biocompatibility. Various polymers, including natural, synthetic, and hybrid polymers, have been used to create hydrogels through chemical or physical crosslinking. Synthetic hydrogels, in particular, have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mechanical properties, mimicking the extracellular matrix (ECM) for cell growth and tissue formation. The article discusses strategies for designing synthetic hydrogels with ECM-mimetic properties, such as cell adhesion, proteolytic degradation, and growth factor binding. It also highlights the importance of controlling the network structure of hydrogels to ensure proper degradation, diffusion of bioactive molecules, and cell migration. The article concludes with a discussion on the challenges and future perspectives in the design and synthesis of bioactive or biomimetic hydrogels for tissue engineering applications.