Plants which can accumulate very high concentrations of heavy metals and metalloids are called hyperaccumulators. The bioconcentration factor – which is calculated as the ratio of the concentration of an element in a plant to its concentration in medium – as well as the translocation factor – which is the ratio of the concentration of the element in its aerial parts to that in roots – must be larger than unity in hyperaccumulators. Several strains of endophytic bacteria isolated from plants also show high heavy metal accumulation ability. Freshwater invertebrates, especially mollusks and crustaceans, are efficient accumulators of heavy metals and metalloids. Differences in metal accumulation can be observed among the different organs or tissues, as well as between the same species occupying different habitats. Organ- or tissue-specific accumulation of heavy metals and metalloids is also seen in aquatic insects and fish species. Many commercially important and widely consumed food fishes all over the world have been found to accumulate arsenic and several heavy metals such as mercury to concentrations that may prove to be hazardous to human health. Reservoirs in polluted areas can become storehouses of toxic pollutants including heavy metals.