Mannosylerythritol lipids (MELs) are surface-active compounds synthesized by yeast strains of the genus Pseudozyma sp. or Ustilago sp., from soybean oil or n-alkane (over 100g/L). The former strain produces it as the major while the latter as the minor component. They belong to the class of glycolipids. Although MELs have been known for more than five decades, they recently regained attention due to their environmental compatibility, mild production conditions, structural diversity, self-assembling properties, versatile biochemical functions, and high yield when compared to other biosurfactants. In this chapter, structural diversity of MELs, genetic diversity of MEL producers, various fermentation conditions, downstream steps involved in their separation, factors affecting the amount and type of MEL produced, and self-assembling properties and their applications are discussed. The biosynthetic pathways and the genetic regulation of the MEL production are also included. [ABSTRACT FROM AUTHOR]