Charles Leroy Ellenberger is perhaps best known as a one-time advocate, but now a critic, of controversial writer Immanuel Velikovsky and his works on catastrophism. He first read Worlds in Collision in 1969. In 1979, he became a contributing editor to the Velikovsky-inspired Kronos journal, and has contributed material to many other publications. In 1980 he was selected by the editor of Astronomy magazine to debate James Oberg on Velikovsky.

Immanuel Velikovsky was a Russian-American psychoanalyst, writer, and catastrophist. He is the author of several books offering pseudohistorical interpretations of ancient history, including the U.S. bestseller Worlds in Collision published in 1950. Velikovsky's work is frequently cited as a canonical example of pseudoscience and has been used as an example of the demarcation problem.