The debate over the efficacy of bioaugmentation rages on, with research continuing to demonstrate that its advantages for soil bioremediation are difficult to predict; however, when it works, the results are often very encouraging. The difficulties arise from, among others, the diversity of the microorganisms used, environmental heterogeneity, and variations in the influence of critical parameters (e.g. humidity, microbial predation and "bioavailability') which, unfortunately, are not even always identified.