Abstract: Gut microbial population of chicken mainly comprises of gram positive bacteria, most of which are facultative anaerobes from crop to terminal ileum while caeca additionally contain strict anaerobes. In chickens, the main site of bacterial activity are the crop and the caeca and to a lesser extent, the small intestines. Bacterial fermentation of most non-digestible carbohydrates occurs in the crop and caeca resulting in the production of Short-Chain Fatty Acids (SCFAs) which may provide extra energy to the bird. SCFAs can accelerate gut epithelial cell proliferation thereby increasing intestinal tissue weight resulting in changes of mucosal morphology. Certain beneficial bacteria create a microenvironment hostile to other bacterial species by producing antimicrobial metabolites, a process known as competitive exclusion. The digestive tract of birds also contains pathogenic such as Salmonella, Escherichia coli, Clostridium perfringens, etc. which produce harmful substances like endotoxins. These endotoxins cause fever and the release of endogenous pyrogens which act on thermoregulation centres in the hypothalamus. The development of gut and the competitiveness of beneficial and harmful bacteria can be altered by dietary manipulation (enzymes, prebiotics, probiotics, mannan oligosaccharides and symbiotics) which can alter not only gut dynamics but also many physiologic processes due to the end products metabilosed by symbiotic gut flora.