898 Conclusions Pentanols are a useful class of compounds that have garnered increasing attention due to recent pressure to find alternatives to petroleum. While a lot of this attention is toward the fuel applications of petroleum, it should be remembered that petroleum-derived products permeate modern society. Bio- logically produced pentanols can be a part of the solution to moving beyond petroleum, and this review has summarized current methods of pentanol production (Table 1) and discussed many of the diverse ways that pentanols are useful. As far as engineered production in biological systems, production of pentanols is just beginning. While long known as minor fermentation products, engineered production of pentanols has only taken place within the last 2 years. This means that much more research is required before biologically produced pentanols will be industrially relevant. The next major challenge likely to be faced in the microbial production of pentanols is the issue of toxicity. Pentanols are basically short-chain fatty alcohols and thus have a hydrophobic region of significant size. While the large hydrophobic region translates to a low solubility (about 2%) and thus easier separation from water, it also means that these compounds are more fat-soluble and thus tend to associate in the cellular membranes of microbes. There is already much research into the toxicity of hydrophobic compounds in E. coli and other organisms (Okolo et al. 1987; Aono et al. 1994; Sikkema et al. 1995), and some solutions have been studied (Roffler et al. 1987; Aono 1998; Izak et al. 2008). As biological production of pentanols is pushed further, the issue of its toxicity will become more prominent. There are several directions for future research into biologically engineered pentanol production. 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