Evidence continues to accumulate supporting the important role of the gut microbiome in the pathogenesis of multiple chronic illnesses, including neurodegenerative conditions such as Alzheimer’s disease and Parkinson’s disease. Indeed, the gut microbiome affects virtually all of the risk factors for Alzheimer’s disease, from inflammation to insulin resistance to obesity to amyloidogenesis, as well as other factors. The gut microbiomes of patients with Alzheimer’s disease differ from those of non-Alzheimer’s controls. Furthermore, the composition of the microbiome exerts an effect on the timing and extent of pathophysiology, implicating the microbiome as a pathogenetic mediator. Therefore, optimization of the gut microbiome holds promise as a therapeutic approach to Alzheimer’s disease.