In this chapter, the fundamentals of turbulence and stratification are developed in the context of estuarine and coastal flows. We start with a theoretical development of the dynamics of turbulence in sheared, stratified flow, including turbulent lengthscales and energetic, the effects of boundaries, and the efficiency of turbulent mixing. The remainder of the chapter is dedicated to the application of these fundamentals to estuaries, river plumes, and the shelf seas. We categorize estuaries as persistently stratified, periodically stratified, and unstratified, and discuss the structure and variability of turbulent mixing in each case. The direct interaction of freshwater flows and the coastal ocean is considered in the context of mixing in river plumes, where we define the structure of the plume and consider both active and passive mixing of the plume. Finally, mixing in shelf seas, where the stratification may be due to either freshwater influence or thermal structure, we consider both spatial and temporal variability of mixing and stratification.