This methodological appendix for the data feature "What Would It Take to Get Medication for Opioid Use Disorder to More People in Need?" describes the methodologies used to estimate the need for expanded access to buprenorphine and methadone treatment for opioid use disorder (OUD) in Michigan and New Jersey. This research used several data sources--including US Drug Enforcement Administration opioid treatment program data, estimates from the National Substance Use and Mental Health Services Survey, overdose death records from the Centers for Disease Control and Prevention, and data from Michigan and New Jersey state prescription drug monitoring programs--to calculate county-level estimates of OUD treatment needs and the number of additional buprenorphine prescribers required to meet those needs. To estimate treatment need, we calculated estimated rates of past-year OUD by county and averaged two estimates computed using different methodologies that derive from our earlier work and a 2022 methodological paper by Keyes and colleagues.