 New therapies are often trialed using animal models before they are tested on humans in a randomized trial. Animal models also play important roles in establishing whether or not a substance is toxic for humans. The International Agency for Research on Cancer (IARC), for example, classifies substances with respect to the quality of the evidence for their carcinogenicity into five groups. Evidence from animal models is referred to in the characterization of each group (IARC 2006). This raises questions about how such extrapolations from animal models to humans work, and how reliable they are.