The Merck Index is a comprehensive reference book that provides detailed information on chemicals, drugs, and biologicals. It includes data such as chemical properties, structures, and uses, making it a valuable resource for scientists, researchers, and students in various fields, including chemistry and pharmacology. First published in 1889 by the pharmaceutical company Merck & Co., the index has evolved over the years and is now available in both print and digital formats. It serves as an essential tool for identifying substances and understanding their applications in medicine, industry, and research.