The becquerel (symbol Bq) (pronounced: 'be-kə-rel) is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s⁻¹. The becquerel is named for Henri Becquerel, who shared a Nobel Prize with Pierre and Marie Curie in 1903 for their work in discovering radioactivity.

In a fixed mass of radioactive material, the number of becquerels changes with time. Therefore, a sample radioactive decay rate is stated with a timestamp for short-lived isotopes, sometimes after adjustment to some specific date of interest (in the past or in the future). For example, one might quote a ten-day adjusted figure, that is, the amount of radioactivity that will still be present ten days in the future. This can de-emphasize short-lived isotopes.^{[citation needed]} The average human body has 4400 becquerels from decaying potassium-40, which is a naturally-occurring isotope of potassium.

Antoine Henri Becquerel (15 December 1852 – 25 August 1908) was a French physicist, Nobel laureate, and the discoverer of radioactivity along with Marie Skłodowska-Curie and Pierre Curie, for which all three won the 1903 Nobel Prize in Physics.

Becquerel was born in Paris into a family which produced four generations of scientists: Becquerel's grandfather (Antoine César Becquerel), father (Alexandre-Edmond Becquerel), and son (Jean Becquerel). He studied engineering at the École Polytechnique and the École des Ponts et Chaussées. In 1890 he married Louise Désirée Lorieux.

In 1892, he became the third in his family to occupy the physics chair at the Muséum National d'Histoire Naturelle. In 1894, he became chief engineer in the Department of Bridges and Highways.

Becquerel's discovery of radioactivity is a famous example of serendipity, of how chance favors the prepared mind. Becquerel had long been interested in the phosphorescence, the emission of light of one color following a body's exposure to light of another color. In early 1896, in the wave of excitement following Wilhelm Conrad Röntgen's discovery of X-rays the previous fall, Becquerel thought that phosphorescent materials, such as some uranium salts, might emit penetrating X-ray-like radiation when illuminated by bright sunlight. His first experiments appeared to show this.

Marie Skłodowska-Curie (7 November 1867 – 4 July 1934) was a French-Polish physicist and chemist famous for her pioneering research on radioactivity. She was the first person honored with two Nobel Prizes—in physics and chemistry. She was the first female professor at the University of Paris, and in 1995 became the first woman to be entombed on her own merits in the Panthéon in Paris.

She was born Maria Salomea Skłodowska ([ˈmarja salɔˈmɛa skwɔˈdɔfska]) in Warsaw, in what was then the Kingdom of Poland. She studied at Warsaw's clandestine Floating University and began her practical scientific training in Warsaw. In 1891, aged 24, she followed her older sister Bronisława to study in Paris, where she earned her higher degrees and conducted her subsequent scientific work. She shared her 1903 Nobel Prize in Physics with her husband Pierre Curie and with the physicist Henri Becquerel. Her daughter Irène Joliot-Curie and son-in-law, Frédéric Joliot-Curie, would similarly share a Nobel Prize. She was the sole winner of the 1911 Nobel Prize in Chemistry. Skłodowska-Curie was the first woman to win a Nobel Prize, the only woman to date to win in two fields, and the only person to win in multiple sciences.