Soil ingestion is a key exposure pathway in Human Health Risk Assessment for contaminants in soil. The theory and mechanisms of how contaminants in a soil enter the human body through the gastrointestinal tract are outlined. The methods available for measuring human exposure using human, animal and validated in-vitro laboratory methods are described and contrasted. The role of the physico-chemical properties of the soils that control the bioavailability of contaminants are summarised. Finally, examples of how bioavailability/bioaccessibility studies of soils from both anthropogenic and geogenic origin are discussed along with the criteria required for deciding whether bioavailability data should be used in a Human Health Risk Assessment.