Ticks are exclusive blood-feeding parasites that are of medical and veterinary importance. Ticks are also host for several maternally inherited symbiotic bacteria that are non-pathogenic bacteria and have potential roles in tick biology and the transmission of co-infecting pathogens. In order to gain a comprehensive view of these symbionts in ticks, we overviewed their incidence and biological importance within ticks based on available data. The symbionts in ticks are diverse, and their incidence and frequency vary across different tick species and different geographical populations of the same species. In some cases, symbionts of Coxiella, Francisella and Rickettsia genera may provide tick hosts essential nutrients absent from the exclusive food source of ticks and exhibit mutualistic relationships with their hosts. However, most symbionts are facultative and affect the biological phenotypes of their tick hosts through various ways. For some strains of Coxiella and Francisella, advanced genomic data and phylogenetic investigations have revealed their interactions with hosts and their evolutionary transitions of pathogenic and mutualistic forms. These findings are valuable for understanding tick-symbiont associations, and may help to develop new strategies to control ticks and tick-borne diseases.