Ticks of the families Ixodidae and Argasidae are the invertebrate hosts of a variety of human and animal pathogens. The main impetus for developing cell cultures has come from virologists, since viruses depend on their host cells for reproduction and can be studied in vitro only in cells or tissues derived from their vertebrate or invertebrate hosts. Most of the work on tick cell cultures has been with tick-borne arboviruses since ticks are second in importance only to mosquitoes as vectors of arboviruses. But there has been an increasing interest in in vitro studies of tick-borne protozoa such as Babesia and Theileria and of tick-borne rickettsiae in tick cells. The advantages and disadvantages of cell cultures in the study of microorganisms have been discussed by several authors. Apart from enabling one to study the development of pathogens in their invertebrate hosts at a cellular level, cell cultures are an alternative to animal experimentation and offer the prospect of developing vaccines with attenuated virulence based on stages of the pathogen in the vector, particularly the infective stage first encountered by a vertebrate host.