Antibiotic resistance has led poultry nutritionists to find alternatives for antibacterial growth promoters in broilers. Among these substitutes, one is mannan oligosaccharides (MOS), a yeast cell wall derived prebiotic. MOS decreases the load of pathogenic bacteria through 1) binding bacterial type-1 fimbriae 2) increasing goblet cells which produce bactericidal mucin and 3) providing favourable environment for the growth of beneficial bacteria leading to competitive exclusion. Balance between pathogenic and beneficial bacteria causes increase in villus length and decrease in crypt depth which are biomarkers for gut morphological improvement. As structure is equal to function, improvement in intestinal morphology increases activity of digestive enzymes and ultimately improves digestion. Besides these, immunomodulatory effect of MOS activates macrophages of gut associated lymphoid tissues resulting in improvement in cellular, humoral and cutaneous immunity. MOS also increases production of butyric acid and decrease pH of intestine in broilers. Though these combined mechanisms, MOS improves growth rate and performance of broilers.