This review describes the recent development of membrane reactors for biological treatment of waste gases. In this type of bioreactor gaseous pollutants are transferred through a membrane to the liquid phase, where micro-organisms degrade the pollutants. The membrane bioreactor combines the advantages of membrane devices with the clean technology of biological air purification. Two types of membrane materials can be used for gas-liquid contact: hydrophobic microporous material and dense material, such as silicone rubber. Microporous material generally has a higher permeability, but dense membranes can be advantageous in the case specific selectivity is required. Biomass is generally present as a biofilm on the membrane, but may also be suspended in the liquid phase. In a number of cases the reactor performance appears to be hampered by an unstable biofilm performance and/or by clogging of the liquid channels due to excess biomass formation. So far, membrane bioreactors for waste gas treatment have only been tested on laboratory scale. If the long-term stability of these reactors can be demonstrated, we expect membrane bioreactors to be useful tools in the treatment of gas streams containing poorly water-soluble pollutants and highly chlorinated hydrocarbons, which are difficult to treat with the conventional methods for biofiltration.