Kimchi fermentation is carried out by various microorganisms present in the raw materials and ingredients of kimchi. Among them, lactic acid bacteria which can grow in 3% brine play the most active role in the kimchi fermentation; it suppresses the growth of other bacteria which could grow under such conditions. Among the 200 bacteria isolated form kimchi, the important microorganisms in kimchi fermentation are known to be Lactobacillus plantarum, L. Brevis, Streptococcus faecalis, Leuconostoc mesenteroides, and Pediococcus pentosaceus. Most kinds of bacteria belonging to the genus Lactobacillus have been found to be present in kimchi. The number of aerobes increased in the early stage of kimchi fermentation and then decreased, while the number of anaerobes continued to increase during the middle stage. A rapid increase of aerobes in the late stage was due to the growth of film-forming yeasts. Leu. mesenteroides actively grows in the early stage of kimchi fermentation, thereby producing lactic acid and carbon dioxide which could acidify kimchi and create an anaerobic state to suppress the growth of aerobes. Streptococcus actively grows in the early stage of fermentation, Pediococcus in the mid-stage, and L. plantarum and L. brevis in the late stage, which could affect the ripening of kimchi. The rate of kimchi fermentation was markedly affected by salt concentration and fermentation temperature, and kimchi was at an optimum for consumption when it contained 0.6-0.8% titratable acidity (pH 4.2), 3% salt, and high volatile organic acids.