Publisher Summary This chapter reviews the breadth of assays and testing procedures available to assess the safety of a new drug entity. It helps the reader in identifying rapid tests that can detect serious toxicity in molecules at a very early stage. This chapter should yield an understanding of toxicity due to elevated drug exposure (intrinsic toxicity) versus that due to stochastic opportunity (idiosyncratic toxicity) showing how the latter is difficult to predict in drug testing and thus poses a serious risk in the drug development process. In addition to having primary activity and being able to enter the body, access the appropriate tissue and have sufficient target presence to achieve therapeutic utility, a drug must cause no harm to the host and therefore, the third important structure-activity relationship that must be explored for a drug is safety pharmacology. The concept of safety can also be relative. In this chapter, various terminologies related to drug safety are explained. The main aim of safety pharmacology is to define the optimal way in which potentially valuable drugs can be utilized without causing harm.