Over the last three decades organic contaminants have been of increasing importance in environmental monitoring. Dioxins, furans, polychlorinated biphenyls and organochlorine pesticides have determined the environmental research agenda. This has led to an increasing demand for certified reference materials (CRMs). However, CRMs have only been made available in limited numbers, as the production and certification of CRMs is normally a relatively slow process. This paper gives an overview of the available CRMs for biota and sediments for these contaminants and the developments in their quality.