The Net Investment Income Tax (NIIT) is a 3.8 percent surtax on a portion of income above certain thresholds. Congress enacted the NIIT in the Health Care and Education Reconciliation Act of 2010, an amendment to the Affordable Care Act. The NIIT partly mirrors the payroll taxes used to fund Medicare’s Hospital Insurance trust fund, also assessed at a 3.8 percent rate above an income threshold. In this brief, we describe the NIIT and the context for the NIIT in Medicare financing and taxing of households with higher incomes. We describe historical data on revenues generated by the NIIT. We also discuss two recent policy proposals that seek to expand the NIIT and redirect the revenue to the Hospital Insurance trust fund, as opposed to the general revenues that support Supplemental Medical Insurance within Medicare and other government programs. Finally, we discuss some pros and cons of raising additional revenue through the NIIT through the lens of five public financing principles.