History teachers are frequently urged to use primary sources in their classrooms. Yet little research exists to guide them, for history has been virtually ignored by researchers interested in cognition and instruction. The present study explored how people evaluate primary and secondary sources when considering questions of historical evidence. A group of working historians and high school seniors "thought aloud" as they reviewed a series of written and pictorial documents about the Battle of Lexington. Differences were found in how each group reasoned about historical evidence. It is suggested that these differences are due in part to beliefs that frame the act of historical inquiry.