Mass balance analysis, and type of reactors and their behavior are used to design treatment plants and evaluate process performance. The objectives of this chapter are to present: The principles of mass balance analysis in wastewater treatment, flow regime, and types of reactors: batch, continuous-flow stirred-tank reactors (CFSTERs) and plug flow reactors (PFRs), comparative performance of CFSTRs and PFRs, performance of CFSTERs in series, and performance of PFRs with dispersion and conversion, and finally equalization of flow and mass loadings. The law of conservation of mass states that mass can neither be created nor destroyed. Mass balance analyses are routinely used in environmental engineering. To apply mass balance analysis, it is necessary to establish a system boundary, which is an imaginary barrier drawn around the system. The system boundary may surround a node (junction), reactor, container, or a process diagram. Proper selection of system boundary is extremely important to identify all flows and masses into and out of the system.1,2 The generalized mass balance is expressed by the following statement and equations (Equations 3.1 and 3.2a).