Environmental DNA (eDNA) is any DNA found in the environment. Generally, this refers to DNA in water or soil, but sometimes is used to refer to DNA in scat or shed hair samples. Theoretically, eDNA could also refer to DNA found in air samples. Sometimes eDNA is used to refer to the DNA of micro-organisms, where the environmental sample contains the whole organism, but more often it is used to describe DNA of macro-organisms. For the purposes of this website, we focus on eDNA as trace DNA from macro-organisms found in water samples. However, we also include information on environmental detection of pathogens affecting aquatic and terrestrial animals and plants.