The importance of feed particle size, as a tool to optimise the utilisation of feed and improve production efficiency in broilers, has attracted increased attention in recent years. Particle size reduction is widely believed to enhance the access of digestive enzymes to substrates because of the increased surface area of feed particles. Fine particles can negatively affect the development of fore-gut (proventriculus and gizzard), which plays an important role in intestinal health and nutrient utilisation. A well-developed fore-gut can be achieved by feeding coarse particles which is associated with improvements in gut motility and health. However, the results of studies considering the effect of feed particle size on upper gastrointestinal tract development have been inconclusive, because of the confounding effects of feed form (mash vs. pellets). Feed particle size influences the development of gastrointestinal tract and bird performance to a greater extent when the broilers are fed mash than pelleted diets. Particle size-reducing properties of the pelleting process may result in suboptimal gizzard development and changes in the intestinal morphology and microbiota profile.