A wide range of modelling tools is now readily available to aid in the design, operation and optimization of wastewater treatment processes.   Mathematical models of the biological treatment process developed slowly from the late 1960s and with a huge leap forward with the introduction of the Activated Sludge Model (ASM in 1989).  This evolved to incorporate nutrient removal processes and simplify the calibration such that ASM3 is now a widely used and respected tool.  Although libraries are freely available that contain components of ASM3 to build treatment models, commercial versions such as GPS-X, Simba and BioWin are more user friendly and thus with a wider usage.  Freeware models such as STOAT are also available with their own user groups. These models can help designers to predict the performance of plants and thus give confidence to the Client that is will prove fit for purpose.  They can also help operators to take cost-effective and timely remedial actions to ensure consistent treatment efficiency whilst meeting discharge consents.  As improved on-line monitoring sensors and instrumentation becomes more readily available and affordable, these models are now being applied to real-time control of the treatment process.  Data from sensors provides the input for the models and the model output can then be used to control, for instance the MLSS and dissolved oxygen concentration in the aeration basin.But as well as modeling the biological processes that occur in the basin, application of computational fluid dynamics (CFD) to modelling, also permits the performance of the physical structure to be modelled and optimized.  The flow velocities within unit processes such as primary and final settlement tanks, whose performance relies both on achieving quiescent conditions within the tank and controlling operation to match the settling properties of the MLSS, can be visualized using CFD.  In this way placement of baffles and other peripheral equipment can be undertaken to minimize these velocities and thus enhance settlement.But these models are sophisticated tools and their successful application requires an understanding of their limitations as well as the input data that is necessary to ensure their effectiveness.  It is the aim of this course to examine those models that have found widespread usage in the UK.  By use of case studies it will explore their effectiveness and seek to understand the information necessary for their successful application.