The Cycadophyta (the cycads proper), comprise the three families Cycadaceae, Stangeriaceae, and Zamiaceae, with approximately 185 extant species in 11 genera. The group is recognized by the structure of its sporophylls, and by having pinnately compound leaves, though the genus Bowenia has doubly compound leaves, and some fossil forms have a simple leaf.
The extinct seed plant group Peltaspermales bears some resemblance to the cycads, and so has been suggested as their sister group. Others have suggested that the cycads may be closer relatives of the Medullosales, based on their similarity in growth pattern, mannoxylic wood, the presence of multiple leaf traces per leaf, and a double vascular system in the seeds. More recently, a fossil has been described with Ginkgo-like foliage but with cycad morphology and pollen structure, suggesting a possible relationship between two taxa that are otherwise quite unlike each other. The cycads' relationship to other living gymnosperms (especially the conifers) is still unclear.
The Bennettitales, another extinct group of seed plants which is sometimes called the Cycadeoids, was at one time classified in the Cycadophyta, but have since been found to be only superficially similar in their overall morphology -- their growth habit and leaf structure are similar to that of the cycads, but their reproductive organs suggest a closer relationship with the flowering plants.
The Cycad Pages are an invaluable source of information on cycads, with coverage of every known species.