The sulfonium compound S-adenosyl-L-methionine (AdoMet, also known as SAM or SAMe, 1) is one of the most versatile biomolecules in nature and one of the most widely used enzyme substrates, second perhaps only to adenosine triphosphate (ATP) (Schemes 9.1 and 9.2). AdoMet (1) may be found in all phyla of life, and is one of the molecules crucial for the existence of living organisms. First identified by Cantoni in 1953 [1, 2] as the active methyl donor formed from L-methionine and ATP, AdoMet (1) proved to be not only the major methyl donor in nature but also to be involved in many other metabolic reactions [3,4]. The versatility of the cofactor is based on the reactivity of the pivotal sulfonium center, which activates the adjacent carbon atoms for nucleophilic attack, deprotonation, and radical formation. This leads to a unique cofactor in which all constituents are used in biochemical reactions. The group transfer reactions result in the release of one of three different thioether products which are more stable than the sulfonium compound, and thereby providing the driving force for these enzyme-catalyzed reactions. These highly preferable thermodynamics of AdoMet-dependent transfer reactions result in a strong preference for this cofactor compared, for example, to other methyl donors such as N5-methyltetrahydrofolate. Due to the electron lone pair in addition to the three different substituents, the sulfonium group is chiral, and the naturally occurring AdoMet (1) is S-configured at sulfur [5].Many other naturally occurring alkylated thioadenosines are derived from AdoMet (1) such as S-adenosyl-L-homocysteine (AdoHcy, also called SAH, 2) and 5 0 -deoxy-5 0 -methylthioadenosine (MTA, 3) (Scheme 9.3), although these compounds and their related 5 0 -thioethers are beyond the scope of this chapter. AdoMet (1) and its derivatives are involved in key metabolic reactions, cell cycle regulation and the storage of epigenetic information, which makes them not only highly interesting for studying biological pathways but also as in-vitro or in-vivo biochemical tools and candidates for diagnostics and therapeutics.