Technical Abstract: Mycotoxins are low molecular weight secondary metabolites produced by fungi. These compounds are toxic, mutagenic, teratogenic, and carcinogenic to animals and humans. Over 300 mycotoxins have been identified, but only those implicated in mycotoxicoses involving humans have been studied in detail with respect to the biochemistry and genetics of their biosynthesis. Research on the natural occurrence, identification and characterization, biosynthesis and genetic regulatory control of mycotoxins, as well as prevention and control of mycotoxin contamination of food and feed gained momentum since the discovery of their toxic effects on human health. Within the last decade, significant progress toward the understanding of several mycotoxins in the world has been made. In this chapter, the genetics and biochemistry of only the most economically significant mycotoxins, aflatoxins, sterigmatocystin, trichothecenes, and fumonisins are summarized and discussed.