In this review, our recent studies on the phase transition of hydrogels and aqueous solutions of thermosensitive homopolymers and copolymers are briefly summarized. On the basis of the concept of cooperative hydration, which is defined as the association-dissociation of water molecules bound to a polymer chain with a positive correlation between adjacent bound water molecules, we theoretically examine the following phenomena: (i) the high-temperature collapse of poly(N-isopropylacrylamide) (PNIPAM) gels in water; (ii) the reentrant volume phase transition of PNIPAM gels in mixed solvents of water/methanol; (iii) the temperature dependence of the swelling behavior of PNIPAM microgels in mixed solvents of water/methanol, the selective solvent adsorption of the gels and the selective solvent binding to the polymer chains; and (iv) the nonlinear depression of the lower critical solution temperature of thermosensitive copolymers in water as the monomer composition is varied. Our theoretical results are compared with the experimental data, and the molecular mechanisms are discussed.