Ticks have been implicated as a source of disease for more than 100 years. Tick-borne diseases are increasingly recognized as important threats to public health. The diversity of human pathogens transmitted by ticks and mosquitoes is similar. Several protozoan, viral and bacterial (including rickettsial) pathogens are transmitted by ticks. Ticks are different from insects such as mosquitoes. We describe the biology of ticks, thereby providing a basis for understanding tick-borne infections. In tick-borne infections, important examples include tick-borne encephalitis, Crimean-Congo hemorrhagic fever, Lyme borreliosis, tick-borne relapsing fever and others. In the parts of these diseases, the emphasis is on clinical diagnosis and treatment.