Robert Osten-Sacken took an interest in insects at the age of eleven through the influence of Joseph N. Schatiloff, a Russian coleopterist. He developed an early interest in entomology specialising in Diptera and especially the Tipulidae. In 1862 Osten-Sacken published, with assistance from Hermann Loew, “Catalogue of the described Diptera of North America” in Smithsonian Miscellaneous Collections, Vol. 3. A later edition of this work appeared in 1878, as Smithsonian Miscellaneous Collections, No. 270. He published many other papers. His work on the Tipulidae included a classification of the family. He also studied insect galls and worked on the Tabanidae. Osten-Sacken corresponded with Hermann Loew, supplying him with specimens, and translated and published Loew's work in the "Monographs of the Diptera of North America", (1862-1873), Smithsonian Miscellaneous Collections, Nos. 6, 171, 219, 256. He proposed the term chaetotaxy.Asteroid335 Roberta is named in his honour. A subspecies of North American snake, Thamnophis sauritus sackeni, was named in his honour by Robert Kennicott in 1859.