Fifteen years ago, Colorado noticeably lagged behind the national average in vaccine uptake. Since then, the state has made considerable progress in improving vaccination rates, collecting and analyzing data to identify high-needs areas, and toward enacting stronger policies to encourage vaccinations. Colorado now exceeds the national average for children who are up-to-date on all recommended vaccines. Despite progress, there is still much room to improve. Infectious diseases remain a source of illness and hospitalization, particularly for Colorado’s youngest and most vulnerable populations. This report, compiled in 2017 by Immunize Colorado, outlines the state of immunizations in Colorado, including the cost of vaccine-preventable diseases, school-required vaccination policies, reasons for under-vaccination and opportunities to reach the under-immunized.