Hydrogels have a three-dimensional crosslinked molecular structure which absorb large quantities of water and swell in a physiological environment.Hydrogels are a class of polymers made from hydrophilic repeat units that interact with water molecules by hydrogen bonding, polar and ionic interaction to take up water many times the initial polymer weight.Further, the polymer chains in the hydrogel are linked via crosslinks to form an infinite network to prevent dissolution of the polymer chains in an aqueous medium.Hydrogels can be natural or synthetic.Due to their high water content, oxygen molecules, nutrients, peptides, proteins, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) biomolecules diffuse readily through hydrogels.Further, cells immobilized in hydrogels maintain their viability and function.As a result of these benefits, hydrogels are used extensively in medical applications for replacement, repair, and regeneration of soft biological tissues.There are >8000 references to hydrogels in PubMed and >15,000 in Web of Science search engines.Recently, hydrogels have been used as a matrix for delivery of cells and morphogens to the site of injury in regenerative medicine.Natural as well as synthetic hydrogels are used in tissue replacement, repair, and regeneration.Natural hydrogels can be derived from plants or animals.Plant-derived hydrogels include polysaccharide-based agarose, alginate, and carboxymethyl cellulose.Animal-derived hydrogels include polysaccharide-based, such as hyaluronic acid, and protein-based, such as collagen, gelatin, chitosan, and fibrin.In particular, injectable and in-situ hardening hydrogels functionalized with photocrosslinkable moieties are very attractive for repairing or regenerating irregularly-shaped tissue injuries using minimally-invasive arthroscopic procedures.In that approach, a suspension of therapeutic cells, morphogens, and growth factors in a functionalized hydrogel precursor solution is injected through a catheter to the injury site guided by imaging.After injection, the precursor solution is hardened or gelled by shinning ultraviolet or visible light enabled catheter.More recently, hydrogels are being used as bioinks for printing cells, morphogens, and growth factors such that the spatial organization of the printed cells and growth factors mimic that of the target tissue.The hydrogel ink in these cellular constructs serves as an extracellular glue to maintain dimensional ability and provide mechanical strength to the construct.The hydrogel also provides ligands for specific interactions between the cell surface receptors and the extracellular matrix (ECM) guide cellular events like adhesion, migration, mitosis, differentiation, maturation, and protein expression.Multiple printing heads are used to print tissue constructs with many cell types and growth factors.The articles in this Special Issue provide exemplary reviews and research works related to the use of hydrogels in tissue engineering and regenerative medicine.Although cells encapsulated in hydrogels maintain their viability and function, the high water content significantly reduces the hydrogel's mechanical strength.As a result, hydrogels unaided cannot be used a matrix for regeneration of load-bearing tissues such as bone.To mitigate this issue, Kumar and collaborators describe in their article titled "A Bioactive Hydrogel and 3D Printed Polycaprolactone System for Bone Tissue Engineering" the development of a novel hard-soft biphasic construct with a gyroid geometry by 3D printing.In this approach, a stiff poly(ε-caprolactone) (PCL) polymer was used to print the hard phase of the construct in a gyroid geometry, whereas a combination of alginate and gelatin was