Open Educational Resources (OER) refer to educational materials like textbooks, images, audio, and video that are licensed openly to be freely used, adapted, and shared. OERs can be revised, reused, remixed, and redistributed under open licenses like Creative Commons. They provide free access and permission for these "4R activities" to dramatically lower the cost of education. While using OER requires more initial work, it allows materials to be updated and personalized to student needs. OER is becoming more feasible as more materials receive peer review and institutions provide support for their creation and use.