Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist who specialised in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.

The stories regarding Huxley's famous 1860 Oxford evolution debate with Samuel Wilberforce were a key moment in the wider acceptance of evolution and in his own career, although some historians think that aspects of the surviving story of the debate are a later fabrication. Huxley had been planning to leave Oxford on the previous day, but, after an encounter with Robert Chambers, the author of Vestiges, he changed his mind and decided to join the debate. Wilberforce was coached by Richard Owen, against whom Huxley also debated about whether humans were closely related to apes.

Huxley was slow to accept some of Darwin's ideas, such as gradualism, and was undecided about natural selection, but despite this, he was wholehearted in his public support of Darwin. Instrumental in developing scientific education in Britain, he fought against the more extreme versions of religious tradition. Huxley coined the term "agnosticism" in 1869 and elaborated on it in 1889 to frame the nature of claims in terms of what is knowable and what is not.

Huxley had little formal schooling and was virtually self-taught. According to Edward Poulton, he became perhaps the finest comparative anatomist of the later 19th century. He worked on invertebrates, clarifying relationships between groups previously little understood. Later, he worked on vertebrates, especially on the relationship between apes and humans. After comparing Archaeopteryx with Compsognathus, he concluded that birds evolved from small carnivorous dinosaurs, a view now held by modern biologists.

The tendency has been for this fine anatomical work to be overshadowed by his energetic and controversial activity in favour of evolution, and by his extensive public work on scientific education, both of which had significant effects on society in Britain and elsewhere. Huxley's 1893 Romanes Lecture, "Evolution and Ethics", is exceedingly influential in China; the Chinese translation of Huxley's lecture even transformed the Chinese translation of Darwin's Origin of Species.

Thomas Henry Huxley was born in Ealing, then a village in Middlesex. He was the second youngest of eight children of George Huxley and Rachel Withers. His parents were members of the Church of England, but he sympathized with nonconformists. Like some other British scientists of the nineteenth century such as Alfred Russel Wallace, Huxley was brought up in a literate middle-class family which had fallen on hard times. His father was a mathematics schoolmaster at Great Ealing School until it closed, putting the family into financial difficulties. As a result, Thomas left school at the age of 10, after only two years of formal schooling.

Despite this lack of formal schooling, Huxley was determined to educate himself. He became one of the great autodidacts of the nineteenth century. At first he read Thomas Carlyle, James Hutton's Geology, and Sir William Hamilton's Logic. In his teens, he taught himself German, eventually becoming fluent and used by Charles Darwin as a translator of scientific material in German. He learned Latin, and enough Greek to read Aristotle in the original.

Later on, as a young adult, he made himself an expert, first teaching himself about invertebrates, and later on vertebrates. He did many of the illustrations for his publications on marine invertebrates. In his later debates and writing on science and religion, his grasp of theology was better than many of his clerical opponents.