Many diseases are caused or influenced by environmental factors. Broadly defined, the term ambient environment encompasses the various outdoor, indoor, and occupa- tional settings in which humans live and work. In each of these settings, the air people breathe, the food and water they consume, and the toxic agents they are exposed to are major determinants of health. Other environmental factors pertain to the individual (“personal environment”) and include tobacco use, alcohol ingestion, therapeutic and “recreational” drug consumption, diet, and the like. It is generally believed that factors in the personal environment have a larger effect on human health than that of the ambient environment, but new threats related to global warming (described later) may change this equation. The term environmental disease refers to disorders caused by exposure to chemical or physical agents in the ambient, workplace, and personal environments, includ- ing diseases of nutritional origin. Environmental diseases are surprisingly common. The International Labor Organi- zation has estimated that work-related injuries and ill- nesses kill more people per year globally than do road accidents and wars combined. Most of these work-related problems are caused by illnesses rather than accidents. The burden of disease in the general population created by nonoccupational exposures to toxic agents is much more difficult to estimate, mostly because of the diversity of agents and the difficulties in measuring the dose and dura- tion of exposures. Whatever the precise numbers, environ- mental diseases are major causes of disability and suffering and constitute a heavy financial burden, particularly in developing countries. Environmental diseases are sometimes the consequence of major disasters, such as the methyl mercury contamina- tion of Minamata Bay in Japan in the 1960s, the leakage of methyl isocyanate gas in Bhopal, India, in 1984, the Cher- nobyl nuclear accident in 1986, the Fukushima nuclear meltdown following the tsunami in 2011, and lead poison- ing resulting from contaminated drinking water in the city of Flint in the United States in 2016. Fortunately, these are unusual and infrequent occurrences. Less dramatic, but much more common, are diseases and injury produced by chronic exposure to relatively low levels of contaminants. It should be noted that a host of factors, including complex interactions between pollutants producing multiplicative effects, as well as the age, genetic predisposition, and dif- ferent tissue sensitivities of exposed persons, create wide variations in individual sensitivity. Disease related to mal- nutrition is even more pervasive. In 2010, it was estimated that 925 million people were malnourished—one in every seven persons worldwide. Children are disproportionately affected by undernutrition, which accounts for more than 50% of childhood mortality worldwide. In this chapter, we first consider the emerging problem of the health effects of climate change. We then discuss the mechanisms of toxicity of chemical and physical agents, and address specific environmental disorders, including those of nutritional origin.