This review presents major aspects of estuarine mixing. Due to the large amounts of brackish water in estuaries produced by mixing of fresh river discharge and salty ocean water, mixing is one major characteristic of what is an estuary. Mixing is quantified locally as well as on estuary-wide scales. Diagnostics of integrated mixing are given for estuarine volumes bounded by transects as well as surfaces of constant salinity moving with the flow. Examples for real-world estuaries are given.