The International Union of Pure and Applied Chemistry (IUPAC,  /ˈaɪjuːpæk/ EYE-ew-pak or /ˈjuːpæk/ EW-pak) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science (ICSU). The international headquarters of IUPAC is located in Zürich, Switzerland. The administrative office, known as the "IUPAC Secretariat", is located in Research Triangle Park, North Carolina, United States. This administrative office is headed by the IUPAC executive director.

IUPAC was established in 1918 as the successor of the International Congress of Applied Chemistry for the advancement of chemistry. Its members, the National Adhering Organizations, can be national chemistry societies, national academies of sciences, or other bodies representing chemists. There are fifty-four National Adhering Organizations and three Associate National Adhering Organizations. IUPAC's Inter-divisional Committee on Nomenclature and Symbols (IUPAC nomenclature) is the recognized world authority in developing standards for the naming of the chemical elements and compounds. Since its creation, IUPAC has been run by many different committees with different responsibilities. These committees all run different projects which include standardizing nomenclature, finding ways to bring chemistry to the world, and publishing works.

Pure and Applied Chemistry (abb. Pure Appl. Chem.) is the official journal for the International Union of Pure and Applied Chemistry. It is published monthly and contains recommendations and reports, and lectures from symposia.

Chemistry is the science of atomic matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds.

Chemistry is sometimes called "the central science" because it connects physics with other natural sciences such as geology and biology. Chemistry is a branch of physical science but distinct from physics.

The etymology of the word chemistry has been much disputed. The genesis of chemistry can be traced to certain practices, known as alchemy, which had been practiced for several millennia in various parts of the world, particularly the Middle East.

Traditional chemistry starts with the study of elementary particles, atoms, molecules,substances, metals, crystals and other aggregates of matter. in solid, liquid, and gas states, whether in isolation or combination. The interactions, reactions and transformations that are studied in chemistry are usually a result of interaction between atoms, leading to rearrangments in the chemical bonds which hold atoms together. Such behaviors are studied in a chemistry laboratory.

International union may refer to: