Publisher Summary This chapter discusses the genetics of starter bacteria, ranging from plasmid biology to genetic tool development, and leading ultimately to elucidation of the complete genome. Starter cultures used by the dairy industry can be broadly divided into two types, mesophilic and thermophilic, based on their optimum growth temperature. Mesophilic starters have a growth optimum of ∼30°C and are used in the production of Cheddar, Gouda, Edam, Blue and Camembert cheeses. Only three species of the lactic acid bacteria (LAB) group employed as commercial starters are considered mesophilic, Lactococcus lactis subsp, lactis, Lc. Lactis subsp, cremoris and Leuconostoc mesenteroides subsp. cremoris. Of all starter bacteria, Lc. lactis subsp, lactis IL1403 and Lc. lactis subsp, cremoris MG1363 are the most thoroughly investigated at the genetic level and have been the workhorses for studying starter cultures. The sequencing of the Lc. lactis subsp, lactis IL1403 genome has provided the first comprehensive insight into the genetics of lactococcal starters. Strain IL1403 is a derivative of the Streptococcus (now Lactococcus) lactis strain IL594, isolated from a cheese starter culture.