Figure 1 is a transmission electron micrograph of Clostridium ljungdahlii [1]. It, like Acetobacterium woodii [2], was named after someone who made great contributions to the understanding of how anaerobes fix carbon dioxide. C. ljungdahlii was named after Lars Ljungdahl; A. woodii after Harland Wood. I had the good fortune to train under both Lars and Harland and have continued research in this area for approximately fifteen years, learning and gaining greater and greater appreciation for the art of anaerobes. This review introduces the concept of the Eastern and Western branches of the Wood/Ljungdahl pathway. The Eastern branch is important in one-carbon metabolism of all organisms; the Western branch is unique to anaerobic microorganisms that use the Wood/Ljungdahl pathway for fixing carbon dioxide or carbon monoxide to generate cell carbon or for synthesizing acetate (acetogens) or methane (aceticlastic methanogens) to generate energy. In focusing on the biochemistry and enzymology of this pathway, I, unfortunately, must risk leaving the important ecology and physiology of these organisms in the lurch. They are responsible for the turnover of a trillion tons of acetic acid per year. They convert a variety of compounds including sugars, aromatic compounds, and inorganic compounds like CO and H2 and CO2 into acetic acid which is then used by methane producing bacteria to make natural gas. They are important in soil microbiology and, in the biology of organisms that house them in their digestive system, like humans, termites and ruminants like cows and sheep [3–5]. The story of acetogens can be traced back to the ancient times when someone discovered that apple or grape juice could be fermented, first to alcohol and then to acetic acid, producing cider vinegar, which contains 3–6% acetic acid. The word acetum is the Latin word for vinegar. Acetic acid was isolated in pure form by Stahl in 1700. Over the years, it has been found to be an excellent solvent for many organic compounds and some inorganic compounds. It also is essential in the production of cellulose acetate and is used widely in the textile and rubber industries.