Over the past two or three decades, considerable effort has been expended trying to probe the “bioavailability” of trace elements in soils amended with wastes or otherwise enriched in such elements. Although an actual bioassay would seen the direct route to such knowledge, numerous “chemical proxies” have been advanced as alternatives that are (i) cheaper and faster, and/or (ii) more predictive. Examples include simple chemical extractions (traditional soil tests or a variant), sequential extraction schemes of varying elaborateness and, more recently, isotopic dilution techniques and diffusion gradients in thin films (DGT). The focus of this talk is a review of the basic physical and biological processes that underlie the accumulation of trace elements in soil-dwelling organisms. Central to such an understanding is the mechanism of exposure of the organism of interest. Higher plants are unique in their propensity to evapoconcentrate hazardous elements at sensitive sites (root surfaces) through the very necessary practice of transpiration. Subsequent uptake and redistribution among plan organs reflects a net effect of two opposing processes: exposure to elemental excess versus plant homeostasis. In classical plant nutrition, long-term uptake of a nutrient is a function of the intensity parameter (i.e., the concentration of the free, uncomplexed element in soil solution) as well as a quantity parameter (i.e., the labile pool associated with the soil solid-phase that can replenish the soil solution upon depletion). However, this paradigm has always had an implicit bias toward situations where the element of interest is limiting, or nearly limiting, for optimal growth of the organism. In metal-contaminated soils, the more immediate hazards (e.g., phytotoxicity) are likely to be primarily a function of intensity unless the buffer capacity is unusually low. Examples from the literature will be used to illustrate these central concepts.