The nonlinear nature of fluid flow gives rise to a wealth of interesting and beautiful phenomena. Many of these are of fundamental importance in the understanding of lakes, oceans, and the atmosphere because of their role in processes such as transport, the energy cascade, and, ultimately, in mixing. This special issue is intended to bring together researchers interested in the fundamental nature of nonlinear processes in rivers, lakes, oceans, and the atmosphere. Examples include, but are not limited to, nonlinear and solitary waves, wave–current and wave–wave interactions, flow instabilities and their nonlinear evolution, turbulence, frontogenesis, double diffusion and the nonlinear equation of state, convection, and river plumes.