Steroids are a remarkable class of chemical compounds found throughout the animal and plant kingdoms. This class includes sterols, such as cholesterol and ergosterol, bile acids, and some steroid hormones. Steroids present a relatively rigid common structure named gonane formed by four fused alicyclic rings. The oxidation state of the steroid nucleus rings and the presence of different attached functional groups determine the particular biological properties of each steroid molecule, i.e., its biological function. Steroidal compounds play crucial biological roles in different organisms, including cell membrane stabilization and regulation of relevant cellular processes such as cell proliferation and tissue differentiation. Steroidal compounds are also relevant carbon and energy sources for different bacteria and appear to participate in various cellular signaling mechanisms.