The volume represents a lifetime’s work of the author, for many years the Stephen P Timoshenko Distinguished University Professor of Fluid Mechanics of the University of Michigan. The papers selected treat the dynamics of stratified or rotating fluids, internal or surface waves, hydrodynamic stability, jets and plumes, flow in porous media, and certain aspects of hydrodynamics in magnetic or electric fields. When the papers are viewed in perspective, heterogeneity, whether in density, entropy, circulation, viscosity, or in some quantity which can be called magnetic circulation, seems to be a recurring theme in the phenomena investigated. It provides a general framework through which the understanding of the various phenomena is facilitated by the satisfying similarity underlying their seeming diversity.