Much of the emphasis of the chapters in this book is on heavy metal(loid)s as soil contaminants, their bioavailability and possible toxicity to plants, ecosystems, animals and humans. However, many of the heavy metal(loid)s are actually micronutrients, that is they are essential (in small quantities) for the normal growth of plants and/or animals. Copper (Cu), manganese (Mn), molybdenum (Mo), ... [Show full abstract] nickel (Ni) and zinc (Zn) are the heavy metals that are essential for higher plants. For animals and humans, chromium (Cr), Cu, cobalt (Co), Mn, Mo, selenium (Se), vanadium (V) and Zn are the micronutrient heavy metal(loid)s. Iron (Fe), not usually considered a heavy metal is essential for both plants and animals. Several other elements, including arsenic (As), cadmium (Cd), lead (Pb) and tin (Sn) may possibly have an essential role at very low concentrations. This chapter briefly covers the essential functions of these heavy metal(loid)s in plants and/or animals and the significance of relatively low and high available concentrations in soils. Deficiencies and toxicities of micronutrients adversely affect plant and animal health, cause reductions in growth rate (and yield), overt symptoms of physiological stress and, in extreme cases, the death of the plant or animal. In many parts of the world, the adverse effects of deficiencies of essential heavy metal(loid)s are more important economically than toxicities arising from soil contamination.