Linus Carl Pauling FRS (/ˈpɔːlɪŋ/ PAW-ling; February 28, 1901 – August 19, 1994) was an American chemist and peace activist. He published more than 1,200 papers and books, of which about 850 dealt with scientific topics. New Scientist called him one of the 20 greatest scientists of all time. For his scientific work, Pauling was awarded the Nobel Prize in Chemistry in 1954. For his peace activism, he was awarded the Nobel Peace Prize in 1962. He is one of five people to have won more than one Nobel Prize. Of these, he is the only person to have been awarded two unshared Nobel Prizes, and one of two people to be awarded Nobel Prizes in different fields, the other being Marie Skłodowska-Curie.

Pauling was one of the founders of the fields of quantum chemistry and molecular biology. His contributions to the theory of the chemical bond include the concept of orbital hybridisation and the first accurate scale of electronegativities of the elements. Pauling also worked on the structures of biological molecules, and showed the importance of the alpha helix and beta sheet in protein secondary structure. Pauling's approach combined methods and results from X-ray crystallography, molecular model building, and quantum chemistry. His discoveries inspired the work of Rosalind Franklin, James Watson, Francis Crick, and Maurice Wilkins on the structure of DNA, which in turn made it possible for geneticists to crack the DNA code of all organisms.

In his later years, he promoted nuclear disarmament, as well as orthomolecular medicine, megavitamin therapy, and dietary supplements, especially ascorbic acid (commonly known as Vitamin C). None of his ideas concerning the medical usefulness of large doses of vitamins have gained much acceptance in the mainstream scientific community. He was married to the American human rights activist Ava Helen Pauling.

Linus Carl Pauling was born on February 28, 1901, in Portland, Oregon, the firstborn child of Herman Henry William Pauling (1876–1910) and Lucy Isabelle "Belle" Darling (1881–1926). He was named "Linus Carl", in honor of Lucy's father, Linus, and Herman's father, Carl. His ancestry included German and English.

In 1902, after his sister Pauline was born, Pauling's parents decided to move out of Portland to find more affordable and spacious living quarters than their one-room apartment. Lucy stayed with her husband's parents in Oswego until Herman brought the family to Salem, where he worked briefly as a traveling salesman for the Skidmore Drug Company. Within a year of Lucile's birth in 1904, Herman Pauling moved his family to Oswego, Oregon where he opened his own drugstore. He moved his family to Condon, Oregon, in 1905. By 1906, Herman Pauling was suffering from recurrent abdominal pain. He died of a perforated ulcer on June 11, 1910, leaving Lucy to care for Linus, Lucile and Pauline.

Pauling attributes his interest in becoming a chemist to being amazed by experiments conducted by a friend, Lloyd A. Jeffress, who had a small chemistry lab kit. He later wrote: "I was simply entranced by chemical phenomena, by the reactions in which substances, often with strikingly different properties, appear; and I hoped to learn more and more about this aspect of the world."

In high school, Pauling conducted chemistry experiments by scavenging equipment and material from an abandoned steel plant. With an older friend, Lloyd Simon, Pauling set up Palmon Laboratories in Simon's basement. They approached local dairies offering to perform butterfat samplings at cheap prices but dairymen were wary of trusting two boys with the task, and the business ended in failure.

At age 15, the high school senior had enough credits to enter Oregon State University (OSU), known then as Oregon Agricultural College. Lacking two American history courses required for his high school diploma, Pauling asked the school principal if he could take the courses concurrently during the spring semester. Denied, he left Washington High School in June without a diploma. The school awarded him an honorary diploma 45 years later, after he was awarded two Nobel Prizes.