I gratefully acknowledge the photograph by Julio Ottino and Paul Swanson used on the cover of the book. This photo had previously appeared in the article "The Mixing of Fluids" by J.M. Ottino, Scientific American, 260, 56-67, 1989. More on this application of dynamical systems is explained in the book "The kinematics of mixing: stretching, chaos, and transport" by J.M. Ottino, Cambridge University Press, 1989.