Microrganisms are routinely ingested by many groups of invertebrates. Organisms that consume plant detritus, decaying fruit, rotting wood, and herbivore dung ingest a complex assemblage of bacteria, protozoa, and fungi along with the plant tissue that constitutes the bulk of their food. Aquatic filter feeders and periphyton scrapers include significant quantities of bacteria and microscopic algae in their diets. Fungal tissue is a major food of many arthropods, including species that graze selectively on the vegetative hyphae growing on dead plant tissue or in the soil and also species that consume the fruiting bodies of macrofungi. There are several groups of insects that go to great lengths to culture the fungus that they consume. Microbial tissues and microbial cells are, of course, potential sources of the various macro- and micronutrients generally needed for normal existence by virtually all animals. Thus, if ingested microorganisms are digested and assimilated, they can contribute to meeting the nutritional requirements of the organism that consumes them. However, in addition to being biomass with potential nutritive value, microorganisms are also active catalytic agents with diverse metabolic capabilities. Thus, if ingested microorganisms survive and proliferate in the digestive tract, or if they liberate enzymes that remaine active in the gut milieu, they can augment or extend the digestive and metabolic capabilites of an organism that consumes them. This paper explores the extent to which the nutritive value and metabolic potential of ingested microorganisms are exploited by invertebrates.