This book is about carcinogenic impurities found in marketed drugs, This book includes the formation pathway of impurities in drugs, its carcinogenic mechanism and an analytical method for its determination. Impurity is an unwanted substance produced in a product during the manufacturing process, storage or also due to contaminated API. This impurity has its own effect which is harmful to product consumers. Sometimes, impurity has carcinogenic and genotoxic potential which leads to DNA damage and induces tumours. In the pharmaceutical industry, many drugs are contaminated with carcinogenic impurity. Because of their carcinogenic effect these drugs are recalled from the market for the safety of consumers. In the past two-year nitrosamine classes impurity are found in many drugs which are recalled from the market.