While the use of pharmaceuticals is always a balance of risks and benefits, the same is not true for impurities in pharmaceuticals; impurities convey only risk. A number of international guidelines and regional guidances instruct drug developers and regulatory agencies on how to evaluate and control impurities in drug substances and drug products. While impurities should always be reduced to the lowest levels that are reasonably practical, it is acknowledged that impurities cannot be reduced to zero and specifications for impurities need to be established. This chapter discusses practical and theoretical methods for qualification of different classes of impurities.