This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides the discussion of medicines as poisons and vice versa. It shows that Galen had extensive experience of poisons and antidotes and wrote about them both at length. The book also shows that among ruling families and elsewhere in Roman imperial times, poisons and suspected poisons were widely used against one’s enemies. It also provides ‘poisoning as politics’, confirms, from a wide range of sources, that poisoning was indeed a political weapon in Renaissance Italy, in the courts of local rulers and of the papacy. The book examines the role of women in preparing or procuring antidotes, a possible further skill in their role as healers. It explores the persistent reliance on, and faith in, the restorative properties of mercury: a known poison used against another poison, the virus of the pox.