This review addresses the role of compounds secreted into the external environment that mediate important aspects of tick behaviour. Known as semiochemicals, these information-containing compounds include pheromones (used for conspecific communication), allomones (defence secretions) and kairomones (used for host identification and location). An impressive body of knowledge has accumulated concerning the identification of the compounds that comprise these semiochemicals. Pheromones are the best known and intensively studied, including arrestment (=assembly) pheromones, attraction-aggregation-attachment (AAA) pheromones and sex pheromones. Arrestment behaviour is mediated by contact with excreta from other ticks. In contrast, aggregating and sexual behaviours comprise a hierarchy of responses to different types of chemical compounds that must be recognized in a sequential order to achieve the end result. Ixodid ticks also produce an allomone that protects against certain insect predators. Finally, ticks use kairomones for host identification, e.g. volatiles such as CO2 and NH3 and various oils such as glandular secretions from deer. Knowledge of tick pheromones has been used to develop innovative new technologies for tick control. These products incorporate tick pheromones and small amounts of pesticide to attract and kill ticks on their hosts or in vegetation. The kairomones and the tick allomone also may be of interest for use in controlling ticks.