This chapter details advancements in radiation therapy between 1915 and 1925, a period significantly influenced by foundational physics discoveries and the work of Marie Curie. Key developments in oncology included the establishment of radiation fractionation principles by Regaud and Coutard and the first descriptions of the oxygen effect. Clinical applications expanded with the use of radon, highlighted by William Duane’s “radium cow”, and innovations in brachytherapy, such as the Paris method. Early teletherapy emerged with devices like Gioacchino Failla’s “radium bomb”, alongside crucial efforts by Failla and Edith Quimby to standardize radiation dosage. The chapter also examines the use of radiation during World War I. Concurrently, a greater understanding of radiation hazards developed, tragically exemplified by the “Radium Girls”, which spurred the creation of professional societies and safety standards.