A contentious bill that would make it more difficult for Californians to avoid mandatory vaccinations for their children leapt into national headlines this week when two high-profile opponents of the bill, the actress Jessica Biel and the anti-vaccination advocate Robert F. Kennedy Jr., visited state lawmakers to lobby against it.

Here’s what you need to know about the debate.

What does current state law say about vaccinations?

California requires every child who attends a public or private school to be immunized against a number of diseases, including measles.

California is one of four states that do not allow parents to opt out of vaccinating their children because of religious or personal beliefs (Maine, Mississippi and West Virginia are the others). But it does allow medical exemptions: A doctor can excuse a child from receiving some or all of the required vaccinations if there is a medical reason to do so.

Under the new bill, doctors could no longer authorize such exemptions on their own; each case would also need to be approved by state health officials.