Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain about their use, or using certain vaccines but not others. The scientific consensus that vaccines are generally safe and effective is overwhelming. Vaccine hesitancy often results in disease outbreaks and deaths from vaccine-preventable diseases. Therefore, the World Health Organization characterizes vaccine hesitancy as one of the top ten global health threats.

"Anti-vaccinationism" refers to total opposition to vaccination; in more recent years, anti-vaccinationists have been known as "anti-vaxxers" or "anti-vax". Vaccine hesitancy is complex and context-specific, varying across time, place and vaccines. It can be influenced by factors such as lack of proper scientifically based knowledge and understanding about how vaccines are made or work, as well as psychological factors including fear of needles and distrust of public authorities, a person's lack of confidence (mistrust of the vaccine and/or healthcare provider), complacency (the person does not see a need for the vaccine or does not see the value of the vaccine), and convenience (access to vaccines). It has existed since the invention of vaccination and pre-dates the coining of the terms "vaccine" and "vaccination" by nearly eighty years. The specific hypotheses raised by anti-vaccination advocates have been found to change over time.

Although myths, conspiracy theories, misinformation and disinformation spread by the anti-vaccination movement and fringe doctors leads to vaccine hesitancy and public debates around the medical, ethical, and legal issues related to vaccines, there is no serious hesitancy or debate within mainstream medical and scientific circles about the benefits of vaccination.

Proposed laws that mandate vaccination, such as California Senate Bill 277 and Australia's No Jab No Pay, have been opposed by anti-vaccination activists and organizations. Opposition to mandatory vaccination may be based on anti-vaccine sentiment, concern that it violates civil liberties or reduces public trust in vaccination, or suspicion of profiteering by the pharmaceutical industry.