This chapter gives a detailed overview on the history, use and toxicology of substances of biological origin such as plant extracts (rotenone, azadirachtin, quassin, anabasin) and fermentation products from soil microorganisms (avermectins and spinosins) that are more or less widely used as insecticides in plant protection products or biocides but may also play a role in public health to prevent spread of infectious diseases or even to cure them.