Ethylene is synthesized from S-adenosylmethionine (SAM), an activated form of methionine (Met). Continuous supply of SAM and hence of Met is necessary for the maintenance of prolonged ethylene synthesis, especially when it occurs at high rates. Met can be replenished through de novo synthesis and through the Yang cycle, which recycles 5′-methylthioadenosine (MTA), the by-product of ethylene synthesis, to Met. This chapter summarizes the pathways and regulation of Met and SAM syntheses with a particular focus on the regulation by ethylene. While the Yang cycle was discovered about three decades ago, not all of the respective genes have yet been identified in any one plant to date. In rice, one of the genes encoding acireductone dioxygenase (ARD) is under immediate control of ethylene whereas in Arabidopsis no known Yang cycle gene is regulated by ethylene, likely reflecting species-specific requirements for ethylene production during development and stress adaptation. The molecular mechanisms by which ethylene synthesis and Met/SAM production are coordinated still await discovery. As ethylene synthesis is a major means to control ethylene levels, unravelling these mechanisms is certainly of interest.