The term "megagroup" is proposed as a formal designation for a rock-stratigraphic unit larger than group. Although comparable in size with series and systems, megagroups are defined in terms of lithology and transect the boundaries of units that are based on time of deposition. Seven megagroups are herein recognized in the pre-Pennsylvanian and one in the Mesozoic and Cenozoic rocks of the Illinois region. The Potsdam sandstone megagroup includes the formations in the basal sandstone succession of Cambrian and earliest Ordovician age. The Knox dolomite megagroup of Cambrian and Ordovician age is in part contemporaneous with the Potsdam and extends from the base of the St. Peter sandstone down only to the base of the Potosi dolomite in northern Illinois but to the base of the Bonneterre dolomite in southwestern Illinois. The Ottawa limestone megagroup is the Middle and Upper Ordovician carbonate sequence above the St. Peter, Glenwood, Aylmer, or Simpson clastics. The Maquoketa shale of Upper Ordovician age does not require megagroup treatment within Illinois, though a megagroup might be useful farther E. where such clastics include Middle and Upper Ordovician and Lower and Middle Silurian rocks. The Hunton limestone megagroup is the major Silurian-Devonian carbonate succession, Knobs megagroup is proposed for the Devonian-Mississippian clastic wedge marking the Acadian orogeny. The Mammoth Cave limestone megagroup is a formal substitute for the "Mississippi lime" of common usage. The Pope megagroup is proposed for the sequence of alternating clastic and limestone deposits of Mississippian age above the Mammoth Cave megagroup. A formal megagroup name appears unnecessary for the dominantly clastic Pennsylvanian rocks of the Illinois region. The Embayment megagroup is proposed for the unconsolidated clastics of Cretaceous and Tertiary age of extreme southern Illinois.