Danish astronomer Ole Rømer (1644-1710) is mainly known for the discovery of the velocity of light, but he was also an important inventor of astronomical instrumentation. He carried out a famous series of astronomical observations during three days and nights in October 1706 from his new observatory west of Copenhagen. They are known as his 'Triduum observations', and because of their high accuracy they came to play a significant role during the 18th century in the first studies of stellar motions. Rømer was well aware of the importance of these observations, and on his deathbed he entrusted his spiritual advisor with a copy of this dataset in order to ensure its publication. Rømer's successor, Peder Horrebow, analysed these observations in great detail and published a book, Basis Astronomiæ, in 1735 with a description of all Rømer's many new astronomical instruments. Nearly all other observations by Rømer were lost in the Fire of Copenhagen in 1728. Ole Rømer’s Triduum Observations is published in three volumes. Volume I describes Rømer's life and work in general and the establishing of the new observatory, and it presents a detailed, modern analysis of the observations. From letters and manuscripts we get a clear impression of Rømer's thoughts, and from the observations themselves we obtain a detailed understanding of his instruments and clocks. Volume II contains a photographic reproduction of Basis Astronomiæ with an English translation of the Latin text (the first ever), while Volume III presents a photographic reproduction of the Triduum manuscript from 1710 wich has never been published in its original form. Claus Fabricius is an astronomer working on the European Space Agency project Gaia at the University of Barcelona, Niels Therkel Jørgensen obtained his PhD at the University of Aarhus on a thesis about Peder Horrebow's unpublished Adytum Astronomiæ, and Chr. Gorm Tortzen is a retired associated professor of classical languages. Ole Rømer’s Triduum vol. I-III is published by the Society for Danish Language and Literature.