UASB reactor has gained a wide popularity in its application to the treatment of variety of wastewaters but problems encountered in the treatment of domestic or low strength wastewaters where granular sludge formation is either difficult or takes a very long time for its development and hence the process requires a long period for start-up . Due to this reason, I got motivated to work upon modelling of granular size development in UASB reactor and access it's performance. Other aspects where a little attention is paid by investigators in the modelling of UASB reactor's performance especially in the treatment of domestic or low strength wastewaters is also focussed in the present book. A simple CSTR model has been tried to judge its suitability in evaluation of dynamic and steady state performance of UASB reactor treating low strength wastewaters using experimental results of different investigators. Simulation results will provide useful guidance to the operators under such situations in the field.