Publisher Summary This chapter focuses on the ticks, a distinct group of exclusively blood-feeding ectoparasites familiar to most people in virtually all regions of the world. Ticks are classified with the class Arachnida, the group that contains the familiar spiders and scorpions. Arachnids have chelicerae, which are appendages with pincerlike or scissorlike cutting edges, instead of mandibles. There is no head or thorax such as occurs in insects. Ticks transmit a greater variety of disease causing pathogenic agents than any group of arthropods, including protozoan, viral, bacterial, and even fungal pathogens. An example is Lyme disease, which is now the most important vector borne disease of humans in the United States, Europe, and Asia. In numerous countries in tropical and subtropical regions of the world, tick-borne diseases of livestock such as babesiosis, theileriosis, and heartwater have made it difficult or impossible to raise domestic animals for food or animal products. Ticks can also cause irritating or even fatal injury to humans and animals because of paralysis, toxicity, or severe allergic reactions to their bites.