Immigrants have been identified as a vulnerable population, but there is heterogeneity in the degree to which they are vulnerable to inadequate health care. Here we examine the factors that affect immigrants' vulnerability, including socioeconomic background; immigration status; limited English proficiency; federal, state, and local policies on access to publicly funded health care; residential location; and stigma and marginalization. We find that, overall, immigrants have lower rates of health insurance, use less health care, and receive lower quality of care than U.S.-born populations; however, there are differences among subgroups. We conclude with policy options for addressing immigrants' vulnerabilities.