The Museum Vrolik collection of the Department of Anatomy and Embryology of the Academic Medical Center, University of Amsterdam, founded by Gerardus Vrolik (1775–1859) and his son Willem Vrolik (1801–1863), consists of more than 5,000 specimens of human and animal anatomy, embryology, pathology, and congenital anomalies. Recently, the collection of congenital anomalies, comprising 360 specimens, was recatalogued and redescribed according to contemporary perspectives. The original descriptions, as far as preserved, were compared with the clinical, radiographic, and magnetic resonance imaging findings. We diagnosed 30 symmetrical conjoined twins (CTs), 11 parasitic CTs, and 16 acardiac twins. Within the group of symmetrical CTs, the following concomitant external anomalies were found in 15 specimens: neural tube defects, holoprosencephaly, cleft lip/ palate, umbilical hernia, omphalocele, cloacal exstrophy, peromelia, polydactyly, and facial abnormalities suggestive of a chromosomal abnormality. We discuss the results in the light of historical and contemporary explanations regarding conjoined twinning, including the opinions of Gerardus and Willem Vrolik and of Louis Bolk, one of their successors. Am. J. Med. Genet. 80:74–89, 1998. © 1998 Wiley-Liss, Inc.