In this qualitative case study the authors explored secondary social studies preservice teachers' abilities to discern the digitized primary resources available to them for historical thinking instruction. The emerging analysis highlights the development of these young teachers' knowledge and understandings of digitized resources as they relate to historical thinking via a pragmatic meter and their pedagogical content knowledge. Using the teacher cognition scholarship of Shulman (2004), the study suggests that the preservice teachers' enumerated knowledge sources are vital in tracing teachers' decisions. (Contains 3 figures.)