Steroids are a large group of organic compounds with the perhydro-1, 2-cyclopentano–phenanthrene nucleus, which consists of four fused rings. Sterols are hydroxylated steroids meaning they are alcohols derived from steroids. Steroids and sterols are widely distributed in nature and are present in bile salts, adrenal-cortical and sex hormones, insect molting hormones, saponins, alkaloids, and some antibiotics. Steroids and sterols differ from each other in two ways: the number, type, and position of the substituents and the number and position of the double bonds in the ring. The global sales of steroids (mainly in the form of systemic sex hormones and corticosteroids) are estimated to amount to billions of dollars. Many steroids and sterols are manufactured through microbial action. This chapter discusses nature and uses of steroids and sterols, sex hormones, corticosteroids, saponins, heterocyclic steroids, manufacture of steroids, types of microbial transformations of steroids and sterols, screening for microorganisms and fermentation conditions for steroid transformation.