There is a growing concern about possible ecotoxicological importance of various classes of emerging contaminants in the environment. Numerous field studies designed to provide basic scientific information related to the occurrence and potential transport of specific classes of emerging contaminants in the environment are being conducted with the aim to identify the sources and points of entry of these contaminants into the environment, and to determine their concentrations in both input streams (i.e., urban and industrial wastewaters) and receiving environment. This chapter summarizes the data regarding the occurrence of emerging contaminants in urban and industrial wastewaters, including some prominent classes such as pharmaceuticals, hormones, illicit drugs, surfactants and their degradation products, plasticizers, and perfluorinated compounds.