The IAWQ Model No. 1 is well accepted and used for dynamic modelling of activated sludge plants. The model contains many parameters. Some of the model parameters depend on the wastewater and/or plant. In some applications, the model has to describe a real plant quite accurately and some of the parameters have to be determined. As a consequence, model tuning or calibration has become an active area of research during the last few years. This contribution briefly reviews recent literature on calibration strategies and methods for assessing parameter identifiability of the Model. Some identifiability results for full scale plants are presented obtained by a combined analysis of the parametric sensitivity and the Fisher information matrix.