One approach to predicting the toxicology of novel compounds is to apply expert knowledge. The field of artificial intelligence has identified a number of ways of doing this, and some of these approaches are briefly described in this chapter. We also examine two expert systems-derek, which predicts a variety of types of toxicology, and star, which predicts carcinogenicity-in some detail. star reasons about carcinogenicity using a system of argumentation. We believe that argumentation systems have great potential in this area, and so discuss them at length.