Critical REACH phases include (i) a requirement to notify by 1 December 2008 any intention to register a specific substance and (ii) a deadline of 1 December 2010 to register substances. Chemicals that are manufactured or imported at volumes of 1000 ton or more per year, CMR (carcinogens, mutagens and reproductive toxins) substances imported/manufactured above 1 ton/year and substances that are toxic to the aquatic environment and which are imported ormanufactured above 100 ton/year are regarded as substances of concern. Such substances are expected to be rigorously controlled. The registration of a substance requires the preparation of an extensive dossier to assess the risk to humans and to the environment presented by commercial use of the substance and to describe how the risk is managed. Key to the preparation of the dossier is access to high quality data to describe the parent substance, its environmental fate and, for a large number of substances, human exposure. Significant resources are needed to produce a comprehensive assessment of use, fate, human and environmental exposure and to assess the associated risk. Priority has therefore been given to persistent, bioaccumulative and toxic substances as these may present the greatest risk to animals and humans.