Theodoor Hendrik van de Velde (12 February 1873, Leeuwarden – 27 April 1937 near Locarno in a plane crash) was a Dutchphysician and gynæcologist who served as Director at the Gynæcological Institute in Haarlem. His 1926 book Het volkomen huwelijk (The Perfect Marriage) made him an instant international celebrity. The book advocated knowledge and sensuality in erotic life. In GermanyDie vollkommene Ehe reached its 42nd printing in 1932 in spite of the fact that it was placed on the list of forbidden books, Index Librorum Prohibitorum by the Roman Catholic Church. In Protestant and social democratic Sweden, Det fulländade äktenskapet was widely known although regarded as pornographic and unsuitable for young readers long into the 1960s.
The English translation, Ideal Marriage: Its Physiology and Technique was the best-known work on its subject for several decades, and was reprinted 46 times in the original edition, selling well over a half-million copies.
He is credited with showing in 1905 that women only ovulate once per menstrual cycle. This contributed to the improvement of calendar-based methods of birth control, and later to the creation of other fertility awareness systems.