Although Medicaid, CHIP, and tax subsidies in the marketplaces share a common method for determining eligibility – known as MAGI – there are several exceptions that apply only to specific circumstances in Medicaid and CHIP. These exceptions, along with significant changes in Medicaid/CHIP eligibility, pre- and post-ACA, add complexity to the implementation of MAGI, both in terms of precisely documenting the business rules needed for system development and in training eligibility workers in states where temporary and less automated strategies are used while systems are under development. This brief covers the basics of MAGI with a focus on how it impacts Medicaid and CHIP. It identifies specific eligibility policies that have created confusion for eligibility workers, consumers, and the navigators and the certified application counselors who provide enrollment assistance. It is intended to be a tool for eligibility workers and assisters. It even includes a worksheet (appendix B) to help determine household size in Medicaid and CHIP.