Textbooks have traditionally been standard sources in the history of science. But they have remained marginal in the formulation of large historiographical questions. They are commonly used as a resourceful tool for their collective multiplicity, but they are more rarely considered in their individuality. They have been valued for their capacity of reflecting knowledge, but seldom for their agency in transforming it. The low status of textbooks in comparison to other genres of science is connected to knowledge hierarchies that are historically contingent. This chapter argues for a greater reflexivity leading to the acknowledgement of textbooks as major sources for historiographical development in history of science. It stresses their value to address questions which are central to scientific practice, and their global potential to connect research in local, national, international and transnational perspective. The chapter provides an introduction to science textbook studies and proposes new avenues for future research.