This chapter describes the need for toxicological information to determine the hazard of chemicals and hence allow for risk assessment. Specifically this chapter introduces the grouping together of similar chemicals into categories to allow for read­across, or interpolation, of toxicity. The history behind read­across, its context as an in silico method in addition to its advantages and disadvantages, as well as the purpose and process of category formation are described. This chapter is supported by an overview of the key literature and publicly available guidance in this area.