Electron, carbon, and nitrogen balances can be thought of as relationships among the time rates of change of the various compounds participating in a fermentation process. As such, they define the minimum number of necessary measurements from which the remaining rates can be determined through the use of the balances. All possibilities, however, are not equivalent, and some of them lead to singularities and solutions of high sensitivity. These possibilities are reviewed in this paper, and suggestions are offered in regard to the combination of rate measurements from which robust biomass estimates can be produced.