This article is the first in a new series, Supporting Family Caregivers in the 4Ms of an Age-Friendly Health System, published in collaboration with the AARP Public Policy Institute as part of the ongoing Supporting Family Caregivers: No Longer Home Alone series. The 4Ms of an Age-Friendly Health System (What Matters, Medication, Mentation, and Mobility) is an evidence-based framework for assessing and acting on critical issues in the care of older adults across settings and transitions of care.