Excavations at Modoc Rock Shelter, Illinois (11R5), between 1952 and 1956 documented the antiquity and cultural sequence of the Archaic period in the midcontinental United States. Some researchers questioned the site's stratigraphic integrity because of apparent inconsistencies between the radiocarbon assays and associated artifacts. Excavations in 1980, 1984, and 1987 documented stratigraphic sequences and produced assays of over 40 additional radiocarbon samples from controlled stratigraphic contexts. This report describes the stratigraphic sequences at Modoc Rock Shelter and presents critical evaluations of the radiocarbon assays. This information is used to summarize the depositional and occupational history of the site. Two related stratigraphic sequences are present, one in the Main Shelter and a separate, physically uncorrelated sequence in the West Shelter. Each sequence includes occupations dating to the Early, Middle, and Late Archaic periods, with age ranges between 9000 and 4000 B.P. Questions about the site's stratigraphic integrity, artifact sequences, and analytical procedures are resolved.