Optimal gut health is of vital importance to the performance of production animals (fish, poultry, swine, cattle). Gut health is key to making the productivity, well-being and sustainability of animal production more efficient. Directly and indirectly, the environment is a powerful regulator of gastrointestinal physiology that decisively influences the functional state of the animal. Production animals reared under conventional conditions of intensive production are subjected to various exogenous and endogenous sources of environmental factors that can impact gut health. Exogenous factors are environmental stressors derived from external sources connected with diet, infectious disease, mycotoxin exposure, climate (heat and cold), management practices, biosecurity level, housing, litter, feed access, quality, and components. Endogenous factors are host-related such as age, sex, genetics, and breed or are made within the host’s body, such as hormones and neurotransmitters, in response to a stimulus, stressor, or trigger. Endogenous factors serve to communicate signals both locally and distantly in the body. Understanding the interactions between the diverse environmental factors and the different physiological characteristics of the gastrointestinal tract allows us to advance the understanding of gut health and the ability to regulate animal production. The spirit of this book is to critically address how the interactions of different environmental stressors, both internal and external, influence the various functions of the gastrointestinal system of production animals and to be able to use the information to advance scientific research as well as improve the use of the productive tools available.