Abstract The Battle of Gettysburg in south-central Pennsylvania was the largest ever fought on American soil and one of the most significant in its consequences. Moreover, more clearly than most, it demonstrates the roles which underlying geology and surface topography can play in military actions. Early Mesozoic happenings produced the rocks underlying and shaping the Gettysburg landscape, which influenced the flow of the battle and thereby impacted the course of American history. Integration of the battlefield’s geological and military aspects, however, has not yet been adequately presented in concise field-guide format, and so doing that is the intent of the present article. Inspired by Brown’s (1962) brief summary of the geology of the Gettysburg bat-tlefield, the present co-authors recently wrote a lengthy guidebook and reissued it for a 2006 Geological Society of America Annual Meeting field trip ( Inners et al., 2006 ). A shorter field guide was also needed for that trip that could be used by many other geologists afterwards; therefore, Cuffey condensed the long guidebook into the article here, assisted particularly by Inners and Fleeger, but drawing on all of the authors’ contributions as well.