The Filarioidea are a superfamily of highly specialised parasitic nematodes. Species within this superfamily are known as filarial worms or filariae (singular filaria). Infections with parasitic filarial worms cause disease conditions generically known as filariasis. Drugs against these worms are known as filaricides.

Filarioidea all are specialised parasites and the definitive host is always a vertebrate, a mammal, bird, reptile or amphibian, but not a fish. The intermediate host is always an arthropod.

Most of Filarioidea parasitise wild species, birds in particular, but some, especially in the family Onchocercidae, attack mammals, including humans and some domestic animals. Conditions that result from parasitism by Onchocercidae include some of the most troublesome diseases of the warmer regions, including river blindness and elephantiasis.

The Filarioidea include several families:

In and about endemic regions filarial diseases have been public health concerns throughout recorded history. Archaeological evidence for elephantiasis for example extends back some 3000 years, by which time apparently it already was no novelty. Currently perhaps some hundreds of millions of people worldwide, mainly in tropical regions, are infected with pathogenic species of filariae. Where the diseases are endemic many times more are exposed routinely to infection. Some victims harbour more than one medically significant infection simultaneously and this can complicate diagnosis and treatment.

Humankind is the definitive host of at least eight species of filariae in various families. Six are particularly significant in medical terms. The ones that mainly occupy lymph vessels and cause conditions such as adenolymphangitis, elephantiasis, and filarial fever are:

Three other medically important parasitic species are:

The other two are less seriously pathogenic but commonly parasitise humans.