Itaconic acid, 2-methylidenebutanedioic acid, is a precursor of polymers, chemicals, and fuels. Many fungi can synthesize itaconic acid; Aspergillus terreus and Ustilago maydis produce up to 85 and 53 g l(-1), respectively. Other organisms, including Aspergillus niger and yeasts, have been engineered to produce itaconic acid. However, the titer of itaconic acid is low compared with the analogous major fermentation product, citric acid, for which the yield is > 200 g l(-1). Here, we review two types of pathway for itaconic acid biosynthesis as well as recent advances by metabolic engineering strategies and process optimization to enhance itaconic acid productivity in native producers and heterologous hosts. We also propose further improvements to overcome existing problems.