The biosynthesis of serine, glycine, and one-carbon (C 1 ) units constitutes a major metabolic pathway in Escherichia coli and Salmonella enterica serovar Typhimurium. C 1 units derived from serine and glycine are used in the synthesis of purines, histidine, thymine, pantothenate, and methionine and in the formylation of the aminoacylated initiator fMet-TRNA fMet used to start translation in E. coli and serovar Typhimurium. The need for serine, glycine, and C 1 units in many cellular functions makes it necessary for the genes encoding enzymes for their synthesis to be carefully regulated to meet the changing demands of the cell for these intermediates. This review discusses the regulation of the following genes: serA , serB , and serC ; gly gene; gcvTHP operon; lpdA ; gcvA and gcvR ; and gcvB genes. Threonine utilization (the Tut cycle) constitutes a secondary pathway for serine and glycine biosynthesis. L-Serine inhibits the growth of E. coli cells in GM medium, and isoleucine releases this growth inhibition. The E. coli glycine transport system (Cyc) has been shown to transport glycine, D-alanine, D-serine, and the antibiotic D-cycloserine. Transport systems often play roles in the regulation of gene expression, by transporting effector molecules into the cell, where they are sensed by soluble or membrane-bound regulatory proteins.