Hydrogels, with their exceptional water retention capabilities and biocompatibility, have emerged as promising materials for a variety of applications in tissue engineering, drug delivery, and soft robotics. The advent of 3D and 4D printing technologies has further expanded the potential uses of hydrogels and hydrogel nanocomposites, enabling the fabrication of complex structures with precise spatial control. This chapter delves into the fundamentals of hydrogels and their nanocomposites, examining their properties, synthesis methods, and suitability for 3D and 4D printing applications. It also highlights recent advancements, discusses challenges, and explores future directions in this rapidly evolving field.