Joseph J. Thomson (1856–1940) was one of the leading physicists at the turn of the last century. In 1906 he was awarded the Nobel Prize in physics for discovering the electron and for his work on the conduction of electricity in gases. Thomson's discoveries raised questions concerning the nature of the atom. He demonstrated that the atom is not the simplest unit of matter; rather, it has a structure. Thomson's atomic theory has informally been called the “plum-pudding” model, but the early history of this expression has not been elucidated. The expression is not in Thomson 1, the seminal paper in which Thomson developed his atomic theory, nor is it in any of his subsequent publications. Although the historian Heilbron 2, 3 has argued persuasively that this expression misrepresents Thomson's theory, it is still mentioned in respectable professional texts (sometimes labeled “inappropriate”): see, e.g., 4-8. As evidence for its occurrence in popular accounts, we found that a Google search for “plum pudding atomic model” yields over 100,000 hits. In other words, the expression is a commonplace and, as we show, it is not simply an artifact of recent popularization. So, what were the circumstances that led to calling Thomson's theory the “plum-pudding” model?