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The intestine is a complex tissue that includes a major immune component. Indeed, the numbers of immune cells found in intestinal tissues exceed the numbers found in the rest of the body. The intestinal lumen also contains huge numbers of microbes, including those that are not normally pathogenic (referred to as commensals) and a wide array of pathogens that either reside in, or use, this site as a means of entry to other tissues. Hence, immune function in the intestine is a complex and tightly regulated process geared toward differentiating pathogenic from apathogenic challenge and limiting the growth or survival of a wide array of pathogens. This chapter considers the organization and components of the immune system in the gut before considering the mechanisms stimulated by, and active against, a range of pathogenic microbes encountered by the avian gut.