Turbulence is a ubiquitous phenomenon in environmental fluid mechanics that dramatically affects flow structure and mixing. Thus, it is very important to form both a conceptual understanding and a quantitative description of turbulent flows. This section looks at the origin and statistical description of turbulence, as well as the structure of a turbulent boundary layer. The notes also focus upon the characterization of rapid turbulent mixing in environmental systems. Sample problems in this chapter test the user's ability to evaluate key flow parameters from turbulent boundary layer profiles, as well as to determine the scales of turbulent diffusivity in environmental flows.