The fractionation of organic matter in the various parts which are used for mathematical modelling is discussed. The fractions include inert soluble, readily biodegradable, rapidly hydrolysable, slowly hydrolysable, biomass and inert suspended material. Methods for measuring are also discussed. Fractionation of biomass in wastewater and in activated sludge is difficult at present, as methods are only partly developed. Nitrogen fractions in wastewater are mainly inorganic. The organic nitrogen fractions are coupled to the organic COD fractions. The fractions of COD, biomass and nitrogen found in a specific wastewater seem to be constant even when concentrations vary. Wastewater input to sewers and the sewer transport system significantly influences the raw wastewater composition at treatment plants.