Some fundamental notions related to the flushing of tidal lagoons are reviewed and some important mixing mechanisms are discussed. It is shown that the characteristics of mixing and flushing in tidal lagoons can be described in various but connected ways, introducing the concepts of time scales and dispersion coefficients. For some simple geometrical configurations formulas for the computation of time scales and dispersion coefficients are given. For complex-shaped tidal lagoons field data are necessary in order to obtain quantitative information on time scales, dispersion coefficients or on the contribution of different mixing processes. The theoretical topics dealt with in this paper are illustrated by field data collected in some tidal basins in the Netherlands.