Open textbooks are a type of Open Educational Resource (OER). They present educators with an alternative to commercial textbooks, afford students and educators permissions granted by open licenses, and reduce student costs. The purpose of this qualitative study is to examine how educators evaluate the quality of open textbooks. We analyzed 954 educator reviews of 235 unique open textbooks. American postsecondary educators authored the reviews between April 2014 and March 2017 and the Open Education Network (OEN; formerly the Open Textbook Network, https://open.umn.edu/otn/collected) and openly published the reviews in the Open Textbook Library (OTL, https://open.umn.edu/opentextbooks/), unedited and with Creative Commons Attribution 4.0 International licenses (CC BY 4.0). Overall, reviewers found the open textbooks to be of sufficient quality for use. The reviews provide insight into educator concerns and interests regarding the quality and characteristics of open textbooks and may support peer educators’ consideration, and authors’ and publishers’ creation and revision, of open textbooks.