The biofim attached to the carrier surface plays an important role in wastewater treatment processes. So revealing the mass transfer properties in the biofilm is helpful to improve the biofilm treatment performance and explore the pollutant removal mechanisms. A review of the research progress on mass transfer in biofilms was made from different aspects, such as structure models, the mass transfer micro-environment, mass transfer characteristics based on a gas-liquid-solid three phase system, as well as mass transfer impact factors including biofilm structure and hydraulic conditions. Future research prospects are also summarized.