The Imperfectly Stirred Reactor (ISR) is a model for strongly recirculating reacting flows where the conditional means of the reactive scalars are assumed to be spatially uniform and statistically stationary. This model is based on the Conditional Moment Closure (CMC) method and involves very simplified equations which can be solved at very little computational cost. The governing parameters in these equations are the averaged probability density functions (pdf), the residence time and the chemical mechanism.