The methylation of the thiol group of homocysteine leading to methionine is a biochemical reaction of particular interest since it represents a crossroad of the action of two vitamins, folic acid and cobalamin, both in bacteria and in animals. This enzymic reaction, its mechanism and its regulation which has been studied in detail in several laboratories is discussed. Another route which does not require cobalamin occurs in bacteria and plants. Bacteria possessing both pathways of methionine synthesis show regulatory interconnections between them. Plants which generally are devoid of cobalamin synthesize methionine solely by the cobalaminindependent pathway the mechanism of which is as yet not fully understood.