Lipochitin oligosaccharides (LCOs) consist of an N-acetylglucosamine backbone containing four to five monosaccharide units. At the nonreducing end of the backbone is an N-linked fatty acyl chain. These molecules are partly secreted and play key roles in plant-root symbioses. Synthesized and accumulated at the inner membrane, they can be considered membrane-associated lipids. In this chapter, we summarize the significant contributions made in the 1990s to the biosynthesis of these compounds in the rhizobium background, where microbial LCOs are known as Nod factors. Later work has provided new perspectives on the time course of their complex biosynthetic pathway, which will be described in detail here. Since Nod factor-like molecules have been shown to be synthesized by mycorrhizae, the current trend is to study their time-clock origin and to expand their inventory in this kingdom. Despite significant efforts, little has been published on their biosynthetic pathways in fungi. However, groundbreaking discoveries concerning the presence, distribution, composition, and function of LCOs throughout the fungal kingdom have been published over the last 10 years and will be summarized at the end of the chapter.