Mathematical modelling of activated sludge systems is used widely for plant design,optimisation, training, controller design and research. The quality of simulationstudies varies depending on the project objectives, finances and expertise available.Consideration has to be given to the model accuracy and the amount of time requiredto carry out a simulation study to produce the desired accuracy. Inconsistent approachesand insufficient documentation make quality assessment and comparison of simulationresults difficult or almost impossible. A general framework for the application ofactivated sludge models is needed in order to overcome these obstacles.The genesis of the Good Modelling Practice (GMP) Task Group lies in a workshopheld at the 4th IWA World Water Congress in Marrakech, Morocco where members ofresearch groups active in wastewater treatment modelling came together to developplans to synthesize the best practices of modellers from all over the world. The mostcited protocols were included in the work: HSG (Hochschulgruppe), STOWA, BIOMATHand WERF.The goal of the group was to set up an internationally accepted framework to deal withthe ASM type models in practice. This framework makes modelling more straightforwardand systematic to use especially for practitioners and consultants. Additionally, it helpsto define quality levels for simulation results, provides a procedure to assess this qualityand assists in the proper use of the models.The framework describes a methodology for goal-oriented application of activatedsludge models demonstrated by means of a concise guideline about the procedureof a simulation study and some illustrative case studies. Case studies give examplesfor the required data quality and quantity and the effort for calibration/validation withrespect to a defined goal.Additional features in Guidelines for Using Activated Sludge Models include a chapteron modelling industrial wastewater, an overview on the history, current practice andfuture of activated sludge modelling and several explanatory case studies. It canbe used as an introductory book to learn about Good Modelling Practice (GMP) inactivated sludge modelling and will be of special interest for process engineers whohave no prior knowledge of modelling or for lecturers who need a textbook for theirstudents. The STR can also be used as a modelling reference book and includes anextended appendix with additional information and details of methodologies.