Being part of fish’s natural diets, insects have become a realistic, sustainable feed ingredient for aquaculture. While insects’ nutritional values have been extensively studied in numerous fish species, their impact on the fish health remains to be fully explored. The intestine is the main organ directly interact with diets and of pivotal importance for the growth, development, and protection against pathogens. A well-functioning, healthy intestine is the key to convert feeds into fish biomass efficiently. In this thesis, two feeding trials, one in freshwater and one in seawater, were conducted to generate new knowledge regarding the effects of insect meal diets on the intestinal function, health, and microbiota in Atlantic salmon. The results showed no clear evidence that insect meal diets containing 15% or 60% black soldier fly larvae meal impaired the intestinal function or health in Atlantic salmon. On the contrary, insect meal diets improved intestinal health status by reducing excessive lipid deposition in the proximal intestine. Despite a four-time difference in the insect meal inclusion level between the two feeding trials, the insect meal diets reproducibly modulated the salmon intestinal microbiota enriching a particular group of bacterial genera dominated by members of the Bacillaceae family. The present thesis work generated new knowledge regarding how diets containing insect meal may influence the intestinal function, health, and microbiota in Atlantic salmon. It strengthens the knowledge basis for applying insect meal in aquaculture at a full scale.