Over the past 35 years, index properties and classification methods have been developed in order to define engineering properties of a rock mass, but understanding the geologic setting and significant engineering geology features at a project site remains a key in the evaluation of tunnel ground behavior. Relations among geology, index properties, and behavior are described and examples provided from tunnel projects in volcanics in Nevada, dolomites and shales in the Midwest and Appalachians, Precambrian intrusives of the Front Range, schists on the Eastern Seabord, and sedimentary sequences in the Uinta Mountains. Tunneling reveals geologic features and behavior that are often difficult to determine in any other way. Conditions causing difficult tunneling or unanticipated behavior are highlighted, and the process of exploring and preparing geotechnical data and baseline information is described.