Abstract The engineering geology of three vehicular tunnels along U. S. Highway 14 and 20 in Park County, Wyoming, near Yellowstone Park, is described. The first stage of the investigation was a preliminary study after the pioneer bore of tunnel 1 and previous to the full excavation of tunnel 1 and boring of tunnels 2 and 3. The final stage of the study consisted of supplementary mapping and recommendations for remedial treatment of relevant geologic conditions following full excavation of the three tunnels. The three tunnels traverse a Precambrian gneissic complex consisting of hornblende-mica schist, granodiorite, and granite pegmatites. The most important influence on rock breakage was three major joint sets and the normal faults encountered in tunnel 1. Adverse geologic conditions required the following remedial treatment: (1) portal structures at all six portals; (2) reinforced concrete lining and drains for the extensive fault zones and where the joint sets created a “blocky” condition; and (3) pneumatic applied mortar (gunite) in all areas not lined with reinforced concrete.