An overview is given on the literature concerning the effect of long-term processes, called aging, on metal availability, and consequences for risk assessment of metal-contaminated soils are discussed. Experiments with freshly prepared metal oxyhydroxides and calcites overpredict the influence of aging because of the initial transformations occurring in these systems. Also, freshly ground clays are not representative of field soils because the surface area exposed for fixation is unrealistically high. Experiments with field soils confirm the hypothesis that metal availability can decrease as a result of aging, especially for metals with a small ionic radius. Although aging also seems to affect metal uptake, it should be noted that some organisms are able to regulate their internal metal concentrations within a narrow range, especially for essential elements. Deficiency of essential metals as well as metal toxicity to plants and invertebrates seem to be related to the environmental availability of these metals. Unfortunately, aging effects are often confounded by differences in soil properties between freshly spiked and historically contaminated soils. Environmental parameters such as temperature, moisture content, drying and rewetting cycles, and pH affect the rate of aging, whereas pH seems to be the sole important parameter affecting the extent of aging.