Polymer gels are sensitive to their environment, such as temperature, solvent, and pH, and swell or shrink with slight changes in these environmental parameters. Since the phenomena of gel swelling, the degree of swelling, and the thermodynamic picture of swelling and shrinking are described in detail in Chap. 4 (Thermodynamics of Gels), this chapter describes the dynamic picture of swelling and shrinking of polymer gels. Starting from the continuum theory of polymer gels, theoretical equations for swelling and shrinking of polymer gels are derived. Then, examples of the application of the theory to various gels are given to enhance the reader’s understanding.