A final-clarifier inlet design for improving the flow conditions and the settling efficiency is described. Numerical simulations and experiments are introduced in order to model and analyze the flow field as well as to evaluate the design criteria. It is demonstrated that inlet structures of primary and secondary clarifiers cannot be optimized by the same procedure. Although primary-clarifier inlet design is mainly concerned with dissipation of kinetic energy, secondary- clarifier inlet design requires the consideration of density effects on the flow field as well as flocculation properties of the activated sludge. The suggested design includes (1) an inlet expanding over the whole width of the tank, positioned at the bottom, and with a relatively low aperture height in order to reduce potential energy flux; (2) a series of angle bars beyond the inlet for dissipation of kinetic energy and decrease of velocity fluctuations; and (3) dimensioning the inlet volume as flocculation chamber. The introduced inlet design must be tested in the future in either full-scale or pilot-scale tanks with activated sludge.