A classification of semienclosed basins is proposed in this chapter by following a tidally averaged momentum balance that compares drivers and modifiers of residual flow. Residual flow drivers are characterized by the nondimensional tidal Froude number, while balancing forces are typified by the Ekman number. Thus, the classification is contained in a parameteric space that considers estuaries, tidal rivers, vertically homogeneous lagoons, and frictionless tidal basins. The scheme can be regarded as the baroclinic tendencies shown by any basin, presented in the abscissa, versus the mixing tendencies, as represented in the ordinate. Also considered are situations in which wind stress competes with (a) density gradients and (b) tidal stress. This additional approach allows a dynamic description of basins where the dynamics are purely frictional, non-frictional (geostrophic), and Ekman-type. This classification includes semienclosed basins beyond estuaries.