Analysis of primary sources has been recognised to have a significant impact on the development of students' historical thinking. This study explored US history teachers' attitudes and practices to using primary sources by interviewing six history teachers and observing 29 history lessons between August and November 2016 in Indiana and California. The main focus of the study was on the views of International Baccalaureate (IB) Diploma Programme and Advancement Placement (AP) history teachers. Results indicated that all teachers valued the use of primary sources and most of them used sources frequently. Notable was the deficiency of source evaluation in AP and Regular history lessons in those schools which did not offer the IB Programme. The IB teachers mentioned source evaluation as a part of the source work throughout, and emphasised the importance of the IB format and other pedagogical support provided by the International Baccalaureate Organization in the use of sources.