Radio-Active Substances is a groundbreaking scientific treatise authored by Marie Curie, first published in 1904. This seminal work presents Curie's pioneering research on radioactivity, a term she herself coined, and documents her meticulous experiments and discoveries that revolutionized the field of physics and chemistry. The book delves into the properties, behaviors, and measurement of radioactive materials, with a particular focus on uranium, thorium, polonium, and radium-elements that Curie and her collaborators isolated and studied in detail. Curie systematically describes the methods she used to detect and quantify radioactivity, including the use of electrometers and photographic plates. She explores the chemical processes involved in isolating radioactive elements from minerals such as pitchblende, and provides detailed accounts of the physical and chemical characteristics of these substances. The text also discusses the spontaneous emission of energy from radioactive materials, the transformation of elements, and the implications of these phenomena for the understanding of atomic structure. Throughout Radio-Active Substances, Curie emphasizes the importance of rigorous experimentation and careful observation, offering insights into the challenges and triumphs of her laboratory work. The book is richly illustrated with tables, data, and experimental results, making it both a scientific resource and a historical document. It stands as a testament to Curie's intellect, perseverance, and her pivotal role in the discovery of radioactivity, which would later earn her two Nobel Prizes. This work not only laid the foundation for future research in nuclear physics and chemistry but also inspired generations of scientists to explore the mysteries of the atomic world.