Besides the liver, rumen is one of the most important components of metabolism in ruminants. However, the microbes residing in the rumen are influenced by several complex factors such as diet, which result in fluctuations in the rumen pH. Rumen pH affects feed intake and feed digestibility, subsequently causing microbial shift in the individual members of microbial community residing in the foregut and hindgut. This in turn causes an increase in lipopolysaccharide concentration, among other factors, in the gut fluid and animal blood. Irrespective of diet fed to animals, Firmicutes would probably be the most dominant in high grain diet while Bacteroidetes are dominant in hay diet, and both have a relative abundance of about 80% or more at times. The shift in microbial population is not limited to adult ruminants alone but also occur in calves. Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria were the most abundant in both hay and concentrate diet of newly weaned calves. Prolonged, depressed pH, causes subacute ruminal acidosis. This leads to compromise in the integrity of both foregut and hindgut of ruminants, eventually causing structural changes in the gut physiology. Furthermore, diet containing C-12:0 and C-14:0, which are medium-chain fatty acids, were toxic to rumen protozoa. Phytochemical content in some plant residues when fed to animals also causes shift in microbial population. Therefore, foregut and hindgut pH stability is important for ruminant health and for optimal productivity.