Most of the currently used probiotics belong to prokaryotic origin. Unlike
prokaryotes, several eukaryotic microorganisms can also be very useful to animal’s
health. Since a long time, eukaryotes are used as single cell protein and/or as
components of food starters for human and animal consumption throughout the
world. Apart from these uses, certain eukaryotic microorganisms are also used as
probiotics since they can withstand the harsh milieu of gut and execute beneficial
effects in host. While bacterial probiotics are common, only limited eukaryotic
probiotics belonging to fungi/moulds/yeasts are used in human and animal
practices. Nowadays interest in eukaryotic probiotics is on the rise and in most of
the cases, their efficacy and usefulness has been confirmed by firm scientific
evidences. Among the eukaryotic probiotics, yeasts especially Saccharomyces
species are dominant and routinely used in a broad range of hosts. This chapter
deals with the occurrence, distribution, taxonomic characterization, and detail
modes of action of eukaryotic probiotics with special reference to yeasts in
human and other animals.While