Classical estuaries are the brackish water bodies that result from turbulent mixing between river water and coastal ocean water, yielding inflowing saline water and outflowing brackish water. In contrast, inverse estuaries transform inflowing ocean water, by means of dominant evaporation, into outflowing hypersaline water. We investigate and quantify the underlying hydrodynamic processes and their interactions, using analytical theory and numerical modelling. For all these types of estuaries, ranging from highly idealized to complex and realistic, we study transformation processes and the resulting exchange flow, including impacts on sediment dynamics. A method we often use is the Total Exchange Flow (TEF) analysis framework which allows to consistently quantify the estuarine circulation and the related estuarine mixing in terms of Knudsen bulk values. We also apply water mass transformation theory to understand exchange and mixing in isohaline coordinates, which supports the interpretation of estuaries and river plumes as one continuous functional unit.