Because the process objective is more appropriately focused on the clarified liquid rather than the thickened underflow, the unit operation discussed herein is referred to as primary clarification and the units themselves as primary clarifiers. The objective of the primary clarifier chapter in WEF’s new Clarifier Manual of Practice is to move away from empiricism and towards something more rational and, at the same time, practical. In meeting this objective, emphasis is placed on understanding primary clarifier performance, focusing on the process objective of primary clarifiers and factors affecting their performance. This understanding is tantamount to good primary clarifier design. Two case studies are given demonstrating the approach.