1669 in science
- Marcello Malpighi publishes Dissertatio Epistolica de Bombyce in London, a study of Bombyx mori which is the first published monograph on an invertebrate.
- Robert Morison publishes Praeludia Botanica, emphasising use of the structure of a plant's fruits for its classification.
- Francis Willughby and John Ray publish "Experiments concerning the motion of sap in trees, made this spring".
- Jan Swammerdam publishes Historia Insectorum Generalis in the Netherlands, explaining the process of metamorphosis in insects.
- Nicolas Steno puts forward his theory that sedimentary strata had been deposited in former seas, and that fossils are organic in origin.
- October 29 – Isaac Newton is appointed Lucasian Professor of Mathematics at the University of Cambridge.
- Rasmus Bartholin publishes his observation of the birefringence of a light ray by Iceland spar (calcite).
Physiology and medicine
- Richard Lower publishes his Tractatus de Corde on the workings of the heart.
- Philosophical Transactions of the Royal Society 4: pp. 963–965.
- Emsley, John (2001). Nature's Building Blocks: an A–Z guide to the elements. Oxford University Press. ISBN 978-0-19-850341-5.
- De solido intra solidum naturaliter contento dissertationis prodromus.
- Palmer, Alan; Veronica (1992). The Chronology of British History. London: Century Ltd. pp. 190–191. ISBN 0-7126-5616-2.
- Bartholin, Erasmus (1669). Experimenta crystalli islandici disdiaclastici quibus mira & insolita refractio detegitur. Copenhagen: Daniel Paulli. English translation: Experiments with the double refracting Iceland crystal which led to the discovery of a marvelous and strange refraction tr. by Werner Brandt. Westtown, Pa., 1959.