Ticks are arachnids which are second only to mosquitoes as vectors of human disease agents. They are many tick-borne diseases worldwide such as Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, anaplasmosis, babesiosis, relapsing fever, and tick-borne encephalitis. This chapter explores the biology, ecology, and disease relationships of ticks, including discussion of the various tick species, where each one occurs, their seasonal activity, what animal hosts they feed on, and the like. Lastly, each of the major tick-borne diseases is presented and discussed, including their geographic distributions, tick vectors, and comments about their treatment, prevention, and control.