The purpose of this letter is to provide an update on the overall status of the Department of Defense's (DOD) implementation of GAO's recommendations and to call DOD's continued attention to areas in which open recommendations should be given high priority. Many of the recommendations in this letter directly address key challenges - such as rebuilding readiness, mitigating cyber threats, and controlling costs - that significantly affect DOD's ability to accomplish its mission. In November 2019, we reported that on a government-wide basis, 77 percent of the recommendations we made 4 years ago were implemented. As of May 1, 2020, DOD's recommendation implementation rate was 64 percent. At that time, DOD had 1,058 open unclassified recommendations. In addition, as of May 1, 2020 DOD had 48 open recommendations that were included in sensitive (for example, For Official Use Only) GAO reports, bringing the total number of open recommendations to 1,106. By implementing these recommendations, DOD could significantly improve its operations and help address its critical challenges.