**Abstract**

Heavy metals are defined in many ways, based on various factors such as density and atomic weight. Some of the heavy metals are essential as nutrients for humans such as iron, cobalt and, zinc in small quantities but are toxic in higher quantities. But few metals, such as lead, cadmium and, mercury are poisonous even in small quantities. The toxicity of heavy metals is depending on concentration,period of exposure and route of exposure. Heavy metal exposure takes place on human beings through inhalation from the atmosphere, intake through drinking water and, ingestion through the skin by dermal contact. The present chapter describes the definition of heavy metals, sources of these heavy metals, toxicity and, their impact on various environmental segments, such as air, water and, soil.

**Keywords:** heavy metals, heavy metal toxicity, sources, exposure, environmental impacts
