In vitro studies of mineral bioavailability are useful because they are faster and less expensive than in vivo studies and because they offer better control over experimental variables. Selection of conditions for in vitro assays of mineral availability is arbitrary. Conditions vary widely among laboratories and with the mineral of interest. Absorption takes place in vivo under conditions that allow ongoing influx and efflux of nutrients and digestive secretions. Conditions in the digestive tract respond to the bulk and nutrient composition of a meal. How does one reproduce these conditions in a closed in vitro system? Correlations between in vivo and in vitro assays may depend on choice of conditions for in vivo studies. Studies of mineral bioavailability from soils have developed to the point that mathematical models have been developed that predict the bioavailability of various minerals to plants under field conditions. Definition of such a model for mineral bioavailability to animals or humans will probably be more difficult to achieve. More information is needed on the nature of nonmineral components of foods and how they interact with minerals.