This mixed method study examined teachers' ratings of 37 field-tested social studies lesson plans that incorporated digital primary sources through a grant from the Library of Congress Teaching with Primary Sources program for K-12 teachers. Each lesson, available in an online teaching materials collection, was field-tested and reviewed by at least three teachers other than the lesson's author. The analysis illustrates that the majority of lessons (70%) utilized PowerPoint to show primary sources and pose questions to students about the primary sources examined. Qualitative analysis identified key factors that impacted lesson implementation and teachers' analysis of student learning. This study raises several implications regarding the design and purpose of lesson plans available online for integrating digital primary sources into P-12 teaching, as well as the design and content of professional development whose purpose is to prepare teachers to integrate digital primary sources in their teaching. (Contains 10 tables.)