This paper summarizes current knowledge about immune responses to vaccines that correlate with protection. It discusses the role of antibodies, cellular immunity, and immune memory in vaccine-induced immunity. Antibodies, particularly those that block infection or bacteremia/viremia, are important correlates of protection. Immune memory, including effector and central memory, is also critical. Cellular immunity can act to kill or suppress intracellular pathogens and may synergize with antibody. For some vaccines, true correlates of protection are not known, but surrogates are used. The paper reviews the correlates of protection for various vaccines, including those against encapsulated bacteria like Haemophilus influenzae, pneumococci, and meningococci, and toxin-producing bacteria such as tetanus and diphtheria. It also discusses vaccines against viruses that replicate in the mucosa and cause viremia, such as influenza, hepatitis, and varicella-zoster viruses. For these viruses, antibodies and cellular immunity both play roles in protection. The paper highlights the importance of defining protection against specific outcomes, such as systemic infection, mucosal infection, or severe disease. The paper also discusses the role of cellular immunity in protection against intracellular bacteria like Mycobacterium tuberculosis and the plague. For viruses transmitted by arthropods, such as dengue and Japanese encephalitis, antibodies and cellular immunity are both important. The paper concludes that while antibodies are often the primary correlate of protection, cellular immunity also plays a critical role, especially in long-term protection. It emphasizes the need for further research to define immune correlates for vaccines, particularly for complex pathogens like HIV.This paper summarizes current knowledge about immune responses to vaccines that correlate with protection. It discusses the role of antibodies, cellular immunity, and immune memory in vaccine-induced immunity. Antibodies, particularly those that block infection or bacteremia/viremia, are important correlates of protection. Immune memory, including effector and central memory, is also critical. Cellular immunity can act to kill or suppress intracellular pathogens and may synergize with antibody. For some vaccines, true correlates of protection are not known, but surrogates are used. The paper reviews the correlates of protection for various vaccines, including those against encapsulated bacteria like Haemophilus influenzae, pneumococci, and meningococci, and toxin-producing bacteria such as tetanus and diphtheria. It also discusses vaccines against viruses that replicate in the mucosa and cause viremia, such as influenza, hepatitis, and varicella-zoster viruses. For these viruses, antibodies and cellular immunity both play roles in protection. The paper highlights the importance of defining protection against specific outcomes, such as systemic infection, mucosal infection, or severe disease. The paper also discusses the role of cellular immunity in protection against intracellular bacteria like Mycobacterium tuberculosis and the plague. For viruses transmitted by arthropods, such as dengue and Japanese encephalitis, antibodies and cellular immunity are both important. The paper concludes that while antibodies are often the primary correlate of protection, cellular immunity also plays a critical role, especially in long-term protection. It emphasizes the need for further research to define immune correlates for vaccines, particularly for complex pathogens like HIV.