Geophagy is the deliberate consumption of soil and clay deposits by animals, including man. Various hypotheses have been advanced to explain geophagic behavior, the main ones being: detoxification of noxious or unpalatable compounds present in the diet, alleviation of gastrointestinal upsets such as diarrhea, supplementation of mineral nutrients, and as a means of dealing with excess acidity in the digestive tract. This review focuses on the mineralogical and chemical nature of the materials consumed, and considers whether this has been adequately characterized, particularly in the contexts of the above hypotheses. In many cases, the information provided is fragmentary and a number of inconsistencies and anomalies are identified. In particular, it is suggested that for these materials further information on their mineral constituents is required, particularly with respect to the kaolin minerals, interstratified kaolinite/smectite, the smectitic minerals and iron oxide and aluminous minerals. It is suggested that total chemical analyses of these materials are of little relevance unless supported by a physiologically based extraction test, this approach being essential in any study seeking to confirm the nutrient supplementation hypothesis.