Policy makers and payers increasingly hold health systems accountable for spending and quality for their attributed beneficiaries. Low-value care—medical services that offer little or no benefit and have the potential for harm in specific clinical scenarios—received outside of these systems could threaten success on both fronts. Using national Medicare data for fee-for-service beneficiaries ages sixty-five and older and attributed to 595 US health systems, we describe where and from whom they received forty low-value services during 2017–18 and identify factors associated with out-of-system receipt. Forty-three percent of low-value services received by attributed beneficiaries originated from out-of-system clinicians: 38 percent from specialists, 4 percent from primary care physicians, and 1 percent from advanced practice clinicians. Recipients of low-value care were more likely to obtain that care out of system if age 75 or older (versus ages 65–74), male (versus female), non-Hispanic