The Affordable Care Act's major health insurance expansions and market reforms will take effect in just eight months. In a new blog post, Commonwealth Fund president David Blumenthal, M.D., and vice president for Affordable Health Insurance Sara R. Collins, Ph.D., examine both the reform law's potential to cover millions of people and the groups who will be left out, including people with very low incomes in states that choose not to expand Medicaid and undocumented immigrants.