Building upon research on writing and on students’ development of historical thinking, the current study uses patterns in 427 eighth graders’ writing about sources to assess the sophistication of their historical reasoning. A spectrum is proposed to represent five levels of increasingly sophisticated writing about the source of documents. Using examples of students’ writing, this paper shows how students at lower levels fail to recognize sources of documents or misuse source information. In contrast, students at higher levels of the spectrum critically analyze sources of documents and a few even use source information persuasively in their writing. The spectrum, which categorizes the quality of students’ sourcing, correlates positively with the frequency of sourcing in students’ writing, suggesting a connection between strategy use and historical thinking. Suggestions for designing written assessments of students’ historical thinking are proposed.