The Membrane-Aerated Biofilm Reactor (MABR) is a potentially transformative technology that uses gas-supplying membranes to achieve up to 100-percent oxygen-transfer efficiencies (OTEs), greatly decreasing energy requirements. This research used bench and pilot-scale studies to obtain a fundamental understanding of the MABR technology, and characterize the performance of two types of membranes, ZeeLung and silicone. Results suggest high fluxes can be obtained with both types of membranes, although silicone membranes provided higher fluxes for a given air supply pressure, given their higher mass transfer coefficients. The “venting strategy” allowed closed-end membranes to perform almost identically to open end membranes by periodically opening them to vent back-diffusion gases. Large voids at the base of the MABR biofilm appeared to be the result of protozoan predation. Preliminary pilot scale results show nitrification fluxes somewhat lower than those from the lab, but more tests are needed to confirm.