The letters ‘MMR’ are short for ‘Measles, Mumps, Rubella’. Since the 1980s in the United Kingdom it has been possible to provide immunity to these diseases for a majority of newborn children via vaccination, and national policy to try and achieve this result. This is national policy in other countries too, but the specific vaccine, which combines all of the antibodies into a single product involving a single injection (plus a ‘booster’ after a few years) is less controversial outside the UK. This health risk issue (with autism and related conditions as the possible harm – see below) shares with the mobile phone issue the element of personal choice, but in a complicated way. First, the choice is made not in respect of the individual but in respect of her child: many parents will be more cautious on a child’s behalf than on their own. Second, the personal choice is compromised: if I choose not to vaccinate I am resisting official pressure. Vaccination, with MMR, is the default option. Third, vaccination is offered as a positive health benefit in the first instance, so parents also have to think of the consequences if their unvaccinated child should contract measles, mumps or rubella. Some parents may have come to believe that single vaccines offer a way out of the dilemma. This chapter discusses what is at stake in the MMR debate, then explores websites and Usenet discussion on this subject.