Bioaugmentation is the addition of microorganisms that have the ability to biodegrade recalcitrant molecules in the polluted environment. This approach is less-costly and friendlier to environment compared to the physico-chemical approaches. Wastewater activated sludge contains naturally occurring microorganisms that biodegrade a wide range of pollutants, but as previously mentioned, some pollutants are resistant to biodegradation. Several factors account for this resistance: high toxicity, low water solubility, low bioavailability, high stability and low biodegradability. Some compounds may not be efficiently used as substrates by microbial metabolic enzymes. The chemical structures of certain pollutants may be so complex that consortia of different microorganisms may be necessary for their biodegradation. Bioaugmentation can overcome these challenges, as one of its main advantages is that treatment can be tailored to a specific pollutant that is dominant in the environment. Thus, this approach is attractive for addressing both the increasing number of emerging pollutants as well as pollutants that are present at high concentrations.