Biofuel cells (alternatively termed biobatteries) are devices that convert chemical energy directly into electrical energy in much the same manner as conventional fuel cells.  They are distinguished from conventional fuel cells, however, by the use of biomass (such as carbohydrates, sugar, etc) as a fuel and the use of biocatalysts (enzymes/microbes) for the redox reactions that take place in both the anode and cathode compartment of a biofuel cell.  A biofuel cell can be operated at a variety of pHs and temperatures including neutral pH and ambient temperature, characteristics distinctly different from that of conventional fuel cells.  In this talk, I will provide an overview of biofuel cell research and discuss the concept of biofuel cells in the context of microelectronics and energy harvesting.  Specific emphasis will be on our work at developing an air-breathing biocathode based on detailed investigations (experimental and computational) that reveal that the availability of oxygen is critical to the overall performance of biofuel cells.  Various challenges associated with biocatalyst development, stability, and performance within a biofuel cell will be presented.  I will conclude the talk with results from our recent efforts in the emerging and promising field of microbial fuel cells and highlight some relevant microelectronic applications.