California enacted 3 aggressive laws between 2014 and 2016 in an effort to improve measles vaccination rates. To a large extent these laws were effective in increasing vaccination rates, but some of the improvements were offset by clinicians granting inappropriate medical exemptions for vaccinations. S. Cassandra Pingali, MPH, MS, and Saad B. Omer, MBBS, MPH, PhD, from the Department of Epidemiology at Emory University, Atlanta, Georgia, discuss measles and what happened in California when legislators tried to improve measles vaccination rates. CME will be available on July 2 when the print/online issue of JAMA is published.