The Kinsey scale, also called the Heterosexual-Homosexual Rating Scale, attempts to describe a person's sexual history or episodes of his or her sexual activity at a given time. It uses a scale from 0, meaning exclusively heterosexual, to 6, meaning exclusively homosexual. In both the Male and Female volumes of the Kinsey Reports, an additional grade, listed as "X", was used for asexuality. It was first published in Sexual Behavior in the Human Male (1948) by Alfred Kinsey, Wardell Pomeroy and others, and was also prominent in the complementary work Sexual Behavior in the Human Female (1953). Numbers between 1 and 5 indicate bisexuality.

Introducing the scale, Kinsey wrote:

Today, many sexologists see the Kinsey scale as relevant to sexual orientation but not comprehensive enough to cover all sexual identity issues. They suggest that sexual identity involves at least three different spectra, sexual orientation being only one of them (two others being biological sex and gender identity).

There have been similar studies where the scale is from 0 to 10. In such studies, the person would be asked a question such as "If 0 is completely gay and 10 is completely straight, what is your orientation number?".

Alfred Charles Kinsey (June 23, 1894 – August 25, 1956) was an American biologist and professor of entomology and zoology, who in 1947 founded the Institute for Sex Research at Indiana University, now known as the Kinsey Institute for Research in Sex, Gender, and Reproduction, as well as producing the Kinsey Reports and the Kinsey scale. Kinsey's research on human sexuality, foundational to the modern field of sexology, provoked controversy in the 1940s and 1950s. His work has profoundly influenced social and cultural values in the United States and many other countries.

Kinsey was born on June 23, 1894, in Hoboken, New Jersey, the son of Sarah Ann (née Charles) and Alfred Seguine Kinsey. Kinsey was the eldest of three children. His mother had received little formal education; his father was a professor at Stevens Institute of Technology.

Kinsey's parents were poor for most of his childhood, and the family often could not afford proper medical care. This may have led to a young Kinsey receiving inadequate treatment for a variety of diseases including rickets, rheumatic fever, and typhoid fever. This health record indicates that Kinsey received suboptimal exposure to sunlight (the cause of rickets in those days before milk and other foods were fortified with vitamin D) and lived in unsanitary conditions for at least part of his childhood. Rickets led to a curvature of the spine, which resulted in a slight stoop that prevented Kinsey from being drafted in 1917 for World War I.