Hydrogels are emerging as enabling materials for a wide range of new applications for soft robots because of their flexibility. Some of the polymer chains that make up the hydrogel have a nano-scale function of swelling and contracting in response to external stimuli. In addition, because the network of polymer chains is on the nanometer order, it is possible to encapsulate various functional materials. Therefore, stimulus-responsive hydrogels and functional materials encapsulating hydrogels are also being actively studied for use as soft robot components. From a fundamentals point of view, the nano-scale functions of hydrogels, fabrication method, and integration with functional materials must be considered for each specific application. This section provides a basic understanding of hydrogels and the recent development of novel fabrication and integration of hydrogel with functional materials such as magnetic nanoparticles, Pt catalyst, graphene, photonic colloidal crystal, and living cells.