Publisher Summary This chapter discusses the fermentation and separation methods for the production of several carboxylic acids, followed by a review of extractive fermentation technologies that can alleviate product inhibition and improve fermentation for economical production of many industrially important carboxylic acids that are produced mainly by petrochemical routes. Although many important carboxylic acids can be produced by microorganisms from biomass, with only a few exceptions, the fermentative routes usually cannot compete with petroleum-based chemical synthesis because fermentative production of chemicals usually suffers from a number of serious limitations, mainly low product yield, reactor productivity, and final product concentration, which are caused by severe end-product inhibition. Extractive fermentation to selectively separate the desirable product in situ has the advantages of reducing product inhibition and increasing the fermentation rate and product yield. Also, extractive fermentation with simultaneous back extraction with a base can produce a highly concentrated and relatively pure carboxylate that can be efficiently converted to carboxylic acid by water-splitting electrodialysis. With the oil price exceeding $70 per barrel, it is attractive to produce acetic, butyric, propionic, fumaric, and other carboxylic acids that are currently produced from petroleum-based feedstocks from biomass.