The physical control of water remains one of the most challenging yet overlooked aspects of treatment plant design. Without adequate flow distribution, even the most state-of-the-art treatment processes can be rendered ineffective. This paper presents established basic theory concerning flow distribution and inlet conditions and then shows how to apply these principles to practical designs. Topics include estimating flow distribution among treatment trains and minimizing short-circuiting within basins through the use of turbulent jet theory. Includes 6 references, tables, figures.