As the second of a two part series discussing the evolution of the field of environmental toxicology, this paper presents two case studies: selenium and arsenic. Developments over several decades in the understanding of the behaviour of arsenic and selenium in different chemical forms in various compartments of the environment are discussed. Selenium was initially thought to be toxic, but later investigations showed it to be an essential micronutrient with a variety of biochemical functions, and, importantly, that there is a very narrow gap between the essential and the toxic body burden. Arsenic, on the other hand, has not yet had an essential role established, but enjoys an interesting and notorious history of usage. Arsenic contamination of the drinking water supplies for many millions of people has been a major catalyst for much research into understanding arsenic chemistry in aquifer systems and also arsenic metabolism and toxicity. The relationships between chemical form, bioavailability, toxicity and metabolism of these two semi-metals are being established, especially with use of sophisticated and sensitive analytical instrumentation and biochemical techniques.