ABSTRACT This study will (1) recruit, train and provide resources to approximately 30 Emergency Department (ED) sites throughout the U.S. using implementation facilitation strategies to provide ED-initiated BUP for patients presenting with opioid use disorder (OUD) not receiving medications for opioid use disorder (MOUD). Once implementation is adequately achieved, the sites will (2) conduct a randomized controlled trial (RCT) to compare the effectiveness of sublingual buprenorphine (SL-BUP) versus extended-release buprenorphine (XR-BUP) on ED patients' engagement in formal addiction treatment 7-days after their ED visit. In addition, in an ancillary component of the study, we will (3) assess the use of XR-BUP in ED patients with Clinical Opioid Withdrawal Scale (COS) scores < 8 in a case series to potentially expand the eligibility of patients in the larger RCT to those presenting with little to no opioid withdrawal symptoms. Finally, we will (4) develop and validate ED electronic health record (HER) opioid-related phenotypes, both of which will inform the main RCT.