Section 1115 waivers authorize research and demonstration projects that, in the view of the Health and Human Services (HHS) Secretary, further the purposes of the Medicaid program. The ACA implemented new requirements for these waivers, including that states must have a publicly available, approved evaluation strategy. This brief examines some of the major research questions and hypotheses relevant to the federal and state evaluations of Medicaid expansion Section 1115 waivers and explores key challenges that may hamper research and evaluation efforts.