This pilot study examined middle school and high school student performance on an online historical thinking assessment task. After their teachers received training in the use of digital historical archives, students from all groups engaged in historical thinking behaviors (e.g., observation, sourcing, inferencing, evidence, question-posing, and corroboration) in response to an open-ended document analysis exercise. The types of thinking they did are described, and differences between AP-level and non-AP students are discussed. Challenges teachers face in developing students' historical thinking around visual documents are also discussed. Educators seeking to take advantage of digitized primary source documents need activities with clear curriculum linkages and small exercises that give students guidance in working with different kinds of documents (visual, textual, and audio). In addition, students and teachers need far more practice in learning to make meaning from primary source documents--in beginning to think like historians.