Over the last decades, there has been a large growth in the application of computational fluid dynamics (CFD) to design of water and wastewater treatment facilities. CFD has many advantages over traditional modelling approaches as it is a low-cost, high speed technique for evaluating engineering systems that are difficult to simulate in a laboratory or under field conditions. CFD is able to yield a “virtual prototype” and a good example of this is in its application to the design of compact and more efficient secondary clarifiers for traditional activated sludge plants. CFD can capture the three-dimensional fluid flow inside a clarifier and thus help to minimise turbulence and optimize solids separation. CFD does of course have some drawbacks and users should understand the underlying principles such as the way the codes are constructed, the turbulence concepts and its modelling, the requirements of the input data, accuracy and interpretation of output data and the computational resources required. This course will provide an understanding of the fundamentals of CFD and will permit delegates to fully exploit the potential of the technique yet be aware of its limitations to enable them to interpret more accurately the modelling outputs.