Vaccine adjuvants are a diverse range of compounds that have the ability to improve vaccine potency through more efficient antigen presentation and delivery, stimulation of the innate immune system or prolonged effect on adaptive immunity. Through these mechanisms, adjuvants can enable better vaccine responses in specific population groups, such as babies and the elderly. Adjuvants can also permit using a lower antigen dose while retaining vaccine efficacy, thereby allowing increased vaccination coverage for global needs. Despite their diversity, adjuvants can be divided into several major groups according to their function and chemical composition. The most commonly used adjuvants are aluminum salts, which have been added to human vaccines for nearly a century. Other groups include saponin-based adjuvants, innate stimulators, such as Toll-like receptor agonists, oil and water emulsions, nanoparticles, and more complex adjuvant systems that contain more than one active component, the microbiome as an immunomodulator, and plant-derived substances known as adaptogens.