In 2006, a young girl in Texas became one of the first to receive Gardasil®, a vaccine that would not only protect her from HPV but also dramatically lower her risk of cervical cancer—ushering in a new era of cancer prevention. Today, that same vaccine, now available as Gardasil-9®, is part of a growing arsenal of cancer vaccines approved by the U.S. Food and Drug Administration, proving that vaccines aren’t just for infectious diseases—they can also fight cancer. These groundbreaking tools come in two forms: preventive vaccines that stop virus-linked cancers before they start, and therapeutic vaccines that help the immune system attack existing tumors. With about 15–20% of cancers worldwide tied to viral infections, preventive vaccines like those for HPV and hepatitis B offer a powerful way to reduce cancer risk. Gardasil-9® protects against nine high-risk HPV strains responsible for most cervical, anal, and head and neck cancers, while hepatitis B vaccines—first approved in 1981—help prevent chronic infections that can lead to liver cancer. These vaccines have been administered to millions, quietly reshaping cancer epidemiology. On the treatment side, therapeutic vaccines face a tougher challenge: cancer cells look nearly identical to healthy cells, making them hard for the immune system to detect. Yet science has found a way. Sipuleucel-T (Provenge®), approved in 2010, trains immune cells to target metastatic prostate cancer. Bacillus Calmette-Guérin (BCG), approved in 1990, is used to treat early-stage bladder cancer by sparking a robust immune response in the bladder. And in 2015, Talimogene laherparepvec (T-VEC, or Imlygic®) became the first FDA-approved oncolytic virus therapy, engineered to both kill melanoma cells directly and stimulate systemic immunity. These therapies are not cures for all, but they represent a shift toward personalized, immune-driven cancer care. The success of these five FDA-approved vaccines underscores a profound truth: our immune system, once properly guided, can be a potent ally against cancer. As researchers develop new vaccines targeting tumor-specific mutations and neoantigens, the future holds promise for more effective prevention and treatment strategies that could benefit millions worldwide.