The aim of this invited mini-review is to summarize the rumen transfaunation literature. Rumen transfaunation using the cud from a healthy donor animal to treat a sick recipient animal was practiced long before our understanding of rumen microorganisms. Around the mid-1900 s, scientists began to explore the benefits of rumen transfaunation and the associated microbial populations. Rumen transfaunation has been used clinically to treat indigestion and to enhance the return of normal rumen function following surgical correction of a left-displaced abomasum. Rumen transfaunation was also used to introduce unique rumen microorganisms into animals that were exposed to toxic compounds in plants. Rumen liquor contains chemical constituents that likely contribute to the beneficial effects of re-establishing a normal reticulo-rumen anaerobic fermentation. Recommendations for collecting rumen fluid, storage and volumes transferred are discussed. Rumen transfaunation is a common practice to treat indigestion on dairy and livestock operations. The support of a healthy microbial community in the digestive tract is also used for humans. Fecal microbiota transplantation has been used to treat digestive disorders in humans. Rumen transfaunation, although not widely studied with respect to mode of action, is an effective, practical, and easy method to treat simple indigestion of ruminants.