Originally in family Liliaceae they were then in their own family Agavaceae.
Agavaceae has now been reduced to the subfamily Agavoideae in a loosely defined family Asparagaceae.
Other genera in Agavoideae include Chlorophytum, Furcraea, Hosta and Yucca.

Differences are still seen in the number of genera in the subfamily and the number of species in the
Agave genus (166 to 252) depending on what is included.
Some species are very variable making classification difficult.

Agave americana, A. attenuata and A. sisifolia are seen as garden ornamentals.
A number are known as “Century plant” because of the decades they can take to flower.

Agaves are succulent or semi-succulent plants whose usually flat leaves contain a lot of fibre.
Most have trunks so short they are difficult to see below the leaves but some can develop long trunks.
They typically have a basal rosette of spirally arranged leaves growing from a rhizome (an underground stem).
In some species the branching rhizomes produce pups or offsets that develop into new plants resulting in large dense clumps.

The simple, entire linear, ovate or lanceolate leaves are mostly straight but some are curved, bent or twisted.
There is a long sharp spine at the tip and there are frequently teeth on part or all of the margin.
Leaves are green or bluish, or grey to silvery due to a coating that reduces the evaporation of water.
They often have bands of a deeper or different colour.

One long branched or unbranched inflorescence grows from the centre of the rosette.
In most species the rosette dies after the plant has flowered.
The inflorescences, up to 15 m tall can be massive and may remain erect or droop over due to the weight of the stems and flowers.

Each flower is on a short pedicel with a bracteole at the base.
The perianth has 6 tepals in 2 whorls of three with their bases fused.
The tepals are white, greenish or yellow.

The 6 long stamens extend well past the perianth.
The ovary has 3 locules and a single long style with 3 stigma lobes.
One plant can produce a huge amount of seeds but reproduction is commonly vegetative.
As well as offsets from the rhizomes many produce bulbils in the inflorescence.