The engineering of cells for the production of fuels and chemicals involves simultaneous optimization of multiple objectives, such as specific productivity, extended substrate range and improved tolerance -all under a great degree of uncertainty. The achievement of these objectives under physiological and process constraints will be impossible without the use of mathematical modeling. However, the limited information and the uncertainty in the available information require new methods for modeling and simulation that will characterize the uncertainty and will quantify, in a statistical sense, the expectations of success of alternative metabolic engineering strategies. We discuss these considerations toward developing a framework for the Optimization and Risk Analysis of Complex Living Entities (ORACLE) -a computational method that integrates available information into a mathematical structure to calculate control coefficients. Biofuels and biochemicals: a multi-objective optimization problem Nearly 20 years ago, Tong and Cameron classified the applications of metabolic engineering into five main areas: (i) improved production and utilization of chemicals already produced/used by the host; (ii) extended substrate range for growth and production; (iii) addition of new catabolic activities for the degradation of toxic chemicals; (iv) production of chemicals new to the host; and (v) modification of the cell [1]. In the development of microorganisms for fuels and chemicals, one must consider and optimize almost all of these objectives simultaneously. For example, the economics of the fuels and commodity chemicals will require a host that is able to overproduce a fuel or chemical (native or new to the organism) from a broad range of carbon substrates, some of them not used previously by this organism, with high specific rates and with near-theoretical yields [2]. All of these issues become more challenging when we consider the use of new hosts for engineering [3,4], where there is not enough genomic information, the genetic tools for metabolic engineering are limited, and extensive knowledge about their physiology is lacking.The development of microorganisms for fuels and chemicals will also require the fine-tuning of carbon flows and redox balance. All of the biofuels currently considered require delicate manipulation of the carbon flows in the central metabolism, and any intervention in these pathways can have significant implications for the cellular physiology. Redox balance, energy charge and cofactor levels are also important because they are involved in many of the pathways for the production of biofuels and acids. For example, these pathways involve reactions that produce and consume redox potential, and their directionality is determined by the NAD + /NADH ratio. Metabolic engineering of these pathways could alter the balance of redox and coenzyme cofactors, thereby reducing the productivity and the yield, leading to the production of byproducts that could increase the cost of downstream processing [2,[5][6][...