Membrane transport of nonessential toxic heavy metals (type D heavy metals) not only controls their access to intracellular target sites but also helps determine their uptake, distribution, and excretion from the body. The critical role of membranes in the toxicology of class D metals has attracted the attention of many investigators, and extensive information has been collected on the mechanism(s) of metal transfer across membranes. Characteristics of metal transport in different cells, or even on opposite sides of the same cell, or under different physiological conditions, are not identical, and no unitary hypothesis has been formulated to explain this process in all cells. However, it seems possible that the mechanisms proposed for different cells represent variations on a few common themes.