In terms of its impact on human health, arsenic is unique in that most of the evidence linking it to diseases comes from epidemiological work; animal studies have not provided good models. It is also unique in causing a large number of different damaging effects and, as more studies are conducted, more such effects are found. To date, we know that arsenic from drinking water can cause severe skin diseases including skin cancer; lung, bladder, and kidney cancers, and perhaps other internal tumors; peripheral vascular disease; hypertension; and diabetes. It also seems to have a negative impact on reproductive processes (infant mortality and weight of newborn babies). The toxicology of arsenic involves mechanisms that are still not completely understood, but it is clear that a number of factors can affect both individual and population-level susceptibility to the toxic effects of arsenic-contaminated drinking water. Current research is addressing some of these, including genetic susceptibility and lifestyle factors that may increase arsenic's toxic effects, such as smoking, diet, and concurrent exposure to other substances. The reversibility of some effects upon cessation of exposure is also being investigated.