Sponsored by the American Institute of Mathematics, this initiative was created in 2010 to promote the use of open-access textbooks in the mathematics community and to support the creation of high-quality Open Educational Resources (OER). The Initiative has developed criteria for effective OER textbooks, reviewed over 150 OER textbooks, and approved about 60 that meet the criteria, including all four PROTEUS textbooks. The list of approved textbooks covers nearly all the standard undergraduate mathematics curriculum,and is regularly consulted by faculty. To our knowledge, this is the only curated list of OER textbooks whose offerings are reviewed by a panel of discipline-specific experts. Visit the Open Textbook Initiative.