The high costs of upgrading treatment plants for either nitrification or nutrient removal has spurred many investigators to examine bioaugmentation as a means to increase nitrification rates and thereby reduce the cost (and space requirements) for nitrification. In this paper, two types of bioaugmentation schemes are reviewed, external and in situ. The advantage of external bioaugmentation schemes is that the promotion of nitrification within the mainstream process can be decoupled from its aerobic SRT. The advantage of in situ schemes is that there is less concern about the loss of activity of the seed nitrifiers when transferred to the mainstream process. The literature is reviewed on all process types, and new information is provided on the Bioaugmentation R (BAR) process which has seen the broadest full-scale application of any bioaugmentation scheme. The various forms of bioaugmentation are compared and contrasted and the practical limitations in their implementation are reviewed.