Ingestion is the process of taking in a substance by an organism, usually through the mouth into the gastrointestinal tract, such as through eating or drinking. In single-celled organisms, ingestion takes place by absorbing a substance through the cell membrane. Besides nutritional items, substances that may be ingested include medication (where ingestion is termed oral administration), recreational drugs, and substances considered inedible, such as foreign bodies or excrement. Ingestion can also refer to a mechanism picking up something and making it enter an internal hollow of that mechanism.

Ingestion is a common route taken by pathogenic organisms and poisons entering the body. Some pathogens are transmitted via ingestion, including viruses, bacteria, and parasites, and most commonly, this takes place via the fecal-oral route. An intermediate step is often involved, such as drinking water contaminated by feces or food prepared by workers who fail to practice adequate hand-washing, and is more common in regions where untreated sewage is common.

Ingestion can also refer to the act of taking in something for or as if for digestion. For example, the ingestion of cranberry products may decrease the number of symptomatic urinary tract infections in women. Magnesium oil can be applied topically or ingested.

Ingestion is also a term used in aeronautics to describe the process of drawing foreign matter into the inlet of a jet engine, often causing damage to the engine.

Ingestion is a crucial process in the digestive system of animals, where food is taken into the mouth, prepared in the form of bolus, mixed with saliva, and swallowed.