Wastewater characterization is now regarded as an indispensable step yielding all the necessary information for a reliable modelling and design of biological treatment processes. It should mainly include fractionation of the chemical oxygen demand (COD), and assessment of significant kinetic and stoichiometric coefficients. COD fractionation involves identification of inert and biodegradable COD together with readily biodegradable and slowly biodegradable fractions. Experimental methods developed or selected for the assessment of COD fractions should be compatible with the mathematical models defining biological treatment and should yield consistent and reliable values. A critical review of available experimental methodology is provided and values of significant COD components determined in this work and reported in the literature, for different types of domestic and industrial wastewaters are outlined.