This review revisits the works of Toyoichi Tanaka on the physics of polymer gels and discusses their scientific significance with the keywords of volume phase transition, structure, dynamics, kinetics and inhomogeneities, followed by some recent topics including defect-free homogeneous gels. Then, the modern physics of polymer gels will be considered from the viewpoints of cross-linking, networking, and percolation, along with the scope of future directions of polymer gels and polymer networks.