Many scientists and philosophers of science have described evolution as fact and theory, a phrase which was used as the title of an article by Stephen Jay Gould in 1981. He describes fact in science as meaning data, not absolute certainty but "confirmed to such a degree that it would be perverse to withhold provisional assent." A scientific theory is a well-substantiated explanation of such facts. The facts of evolution come from observational evidence of current processes, from imperfections in organisms recording historical common descent, and from transitions in the fossil record. Theories of evolution provide a provisional explanation for these facts.

Evidence for evolution continues to be accumulated and tested. The scientific literature includes statements by evolutionary biologists and philosophers of science demonstrating some of the different perspectives on evolution as fact and theory.

Each of the words 'evolution', 'fact' and 'theory' has several meanings in different contexts. Evolution means change over time, as in stellar evolution. In biology it refers to observed changes in organisms, to their descent from a common ancestor, and at a technical level to a change in gene frequency over time; it can also refer to explanatory theories such as Darwin's theory of natural selection which explain the mechanisms of evolution.

Fact can mean to a scientist a repeatable observation that all can agree on; it can mean something that is so well established that nobody in a community disagrees with it; it can also refer to the truth or falsity of a proposition. To the public, theory can mean an opinion or conjecture ("it's only a theory"), but in the scientific world it has a much stronger connotation of "well-substantiated explanation". With this number of choices, people often end up talking past each other, and meanings become the subject of linguistic analysis.