This article critically reviews published literature on the relationship between income inequality and health outcomes. Studies are systematically assessed in terms of design, data quality, measures, health outcomes, and covariates analyzed. At least 33 studies indicate a significant association between income inequality and health outcomes, while at least 12 studies do not find such an association. Inconsistencies include the following: (1) the model of health determinants is different in nearly every study, (2) income inequality measures and data are inconsistent, (3) studies are performed on different combinations of countries and/or states, (4) the time period in which studies are conducted is not consistent, and (5) health outcome measures differ. The relationship between income inequality and health is unclear. Future studies will require a more comprehensive model of health production that includes health system covariates, sufficient sample size, and adjustment for inconsistencies in income inequality data.