Boundary layer processes play a crucial role in establishing the circulation patterns of the oceans and atmosphere, significantly affecting both regional climates and the global climate, as well as the distributions of heat, nutrients, species, pollutants, and more. This Special Issue of Fluids is dedicated to recent advances in the theoretical, numerical, observational (including instrumentation development), and experimental investigation of geophysical/environmental boundary layer processes and how those process may influence regional and global circulation. Topics of interest include, but are not limited to, air-sea interactions and fluxes (including the influence of sea ice), western and eastern boundary currents, bottom boundary processes (including complex topography interactions and sediment motion), and internal boundary layers (such as fronts, shear layers, and thermocline processes).