A fundamental and conceptual description of the microbial system in sewers in terms of wastewater organic matter transformations is presented. The description is examplified being under gravity sewer conditions and a conceptual model including main aerobic transformations is shown. The conceptual model of the microbial system basically agrees with the activated sludge model, (Henze et al., 1987). The model can be extended, e.g. to include anaerobic processes and processes taking place in resuspended sewer sediments under aerobic conditions. The concept for modelling transformations of wastewater during transport in sewers is recommended as the basis for further development.