Ticks are parasitic arachnids of the order Ixodida. They are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, and species, but can become larger when engorged. Ticks are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. The timing of the origin of ticks is uncertain, though the oldest known tick fossils are around 100 million years old, and come from the Cretaceous period. Ticks are widely distributed around the world, especially in warm, humid climates.

Ticks belong to two major families: the Ixodidae, or hard ticks, and the Argasidae, or soft ticks. Nuttalliella, a genus of tick from southern Africa, is the only living member of the family Nuttalliellidae, which represents the most primitive living lineage of ticks. Adults have ovoid/pear-shaped bodies (idiosomas) which become engorged with blood when they feed, and eight legs. Their cephalothorax and abdomen are completely fused. In addition to having a hard shield on their dorsal surfaces, known as the scutum, hard ticks have a beak-like structure at the front containing the mouthparts, whereas soft ticks have their mouthparts on the underside of their bodies. Ticks locate potential hosts by sensing odor, body heat, moisture, and/or vibrations in the environment.

Ticks have four stages to their life cycle, namely egg, larva, nymph, and adult. Ticks belonging to the Ixodidae family undergo either a one-host, two-host, or three-host life cycle. Argasid ticks have up to seven nymphal stages (instars), each one requiring blood ingestion, and as such, Argasid ticks undergo a multihost life cycle. Because of their hematophagous (blood-ingesting) diets, ticks act as vectors of many serious diseases that affect humans and other animals.

Ticks belong to the Parasitiformes, a distinctive group of mites that are separate from the main group of mites, the Acariformes. Whether the two groups are more closely related to each other than to other arachnids is uncertain, and studies often resolve them as not closely related. Within the Parasitiformes, ticks are most closely related to the Holothyrida, a small group of free living scavengers with 32 described species confined to the landmasses that formed the supercontinent Gondwana. The phylogeny of the Ixodida within the Acari is shown in the cladogram, based on a 2014 maximum parsimony study of amino acid sequences of 12 mitochondrial proteins. The Argasidae appear monophyletic in this study.

Ticks belong to four different families. The majority of tick species belong to the two families: Ixodidae (hard ticks) and Argasidae (soft ticks). The third living family is Nuttalliellidae, named for the bacteriologist George Nuttall. It comprises a single extant species, Nuttalliella namaqua, and as such is a monotypic taxon. Nuttalliella namaqua is found in southern Africa ranging from Tanzania to Namibia and South Africa. There is one extinct family, Khimairidae, represented by the fossil species Khimaira fossus, currently regarded as the last common ancestral lineage of the Argasidae and Ixodidae.

Discovery and examination of fossilised ticks have driven understanding of basal Ixodida lineages and the evolutionary history of contemporary taxa. Tick paleobiota have been discovered from the end of the Early Cretaceous onwards, most commonly in amber. The discovery of an argasid bird tick in Late Cretaceous New Jersey amber (Turonian ~94–90 Ma) in 2001 was the first mesozoic record of Parasitiformes. Burmese amber (Cenomanian ~99 Ma) has produced the oldest fossil records, helping to resolve the Khimairidae and Nuttalliellidae through the discovery of extinct Khimaira, Deinocroton, Legionaris and Nuttalliella species, as well as identifying ancient species of the living ixodid genera Amblyomma, Ixodes, Haemaphysalis, Bothriocroton and Archaeocroton. Tick paleobiota is also known from late Albian amber, (~105 Ma) as well as Baltic (~56–34 Ma) and Dominican amber (~40-20 Ma). Phylogenetic analysis suggests that the last common ancestor of all living ticks likely lived around 195 Ma in the Southern Hemisphere, in what was then Gondwana, although other models put the origin of the Ixodida at closer to ~270 Ma.

Almost all contemporary taxa fall into one of the two major tick families. The Ixodidae contain 750 species over 18 genera, characterised by a scutum or hard shield. The Argasidae contain about 220 species over 15 genera. Argasid species have no scutum, and the capitulum (mouth and feeding parts) is concealed beneath the body.

Ticks, like mites, belong to the subclass Acari that lack their primary somatic segmentation of the abdomen (or opisthosoma), rather these parasitic arachnids present a subsequent fusion of the abdomen with the cephalothorax (or prosoma). The tagmata typical of other Chelicerata have developed into the gnathosoma (head), which is retractable and contains the mouthparts, and idiosoma (body), which contains the legs, digestive tract, and reproductive organs. The gnathosoma is a feeding structure with mouthparts adapted for piercing skin and sucking blood; it is the front of the head and contains neither the brain nor the eyes. Features of the gnathosoma include two palps, two chelicerae, and hypostome. The hypostome acts as stabilizer and helps to anchor the tick's mouthparts to the host. The chelicerae are specialized appendages used for cutting and piercing into the host's skin while palps are leglike appendages that are sensory in function.