Sedum is a large genus of flowering plants in the family Crassulaceae, members of which are commonly known as stonecrops. The genus has been described as containing up to 600 species, subsequently reduced to 400–500. They are leaf succulents found primarily in the Northern Hemisphere, but extending into the southern hemisphere in Africa and South America. The plants vary from annual and creeping herbs to shrubs. The plants have water-storing leaves. The flowers usually have five petals, seldom four or six. There are typically twice as many stamens as petals. Various species formerly classified as Sedum are now in the segregate genera Hylotelephium and Rhodiola.
Sedum species are found in four of six major crownclades wthin subfamily Sempervivoideae of Crassulaceae and are allocated to tribes, as follows:
Clades and tribes within Sempervivoideae
Clades containing Sedum, shown in blue
In addition at least nine other distinct genera appear to be nested within Sedum. However the number of species found outside of the first two clades (Tribe Sedeae) are only a small fraction of the whole genus. Therefore the current circumscription, which is somewhat artificial and catch-all must be considered unstable. The relationships between the tribes of Sempervivoideae is shown in the cladogram.
There are now thought to be approximately 55 European species. Sedum demonstrates a wide variation in chromosome numbers, and polyploidy is common. Chromosome number is considered an important taxonomic feature.
Linnaeus originally described 15 species, characterised by pentamerous flowers, dividing them into two groups; Planifolia and Teretifolia, based on leaf morphology. with 15 species, and hence bears his name as the botanical authority (L.). By 1828, de Candolle recognized 88 species, in six informal groups. Various attempts have been made to subdivide this large genus, in addition to segregating separate genera, including creation of informal groups, sections, series and subgenera. For an extensive history of subfamily Sedoideae, see Ohba 1978.
Gray (1821) divided the 13 species known in Britain at that time into five sections; Rhodiola, Telephium, Sedum, (unnamed) and Aizoon. In 1921 Praeger established ten sections; Rhodiola, Pseudorhodiola, Giraldiina, Telephium, Aizoon, Mexicana, Seda Genuina, Sempervivoides, Epeteium and Telmissa. This was later revised in what is the best known system, that of Berger (1930), who defined 22 subdivisions, which he called Reihe (sections or series). Berger's sections were:
A number of these, he further subdivided. In contrast, Fröderströmm (1935) adopted a much broader circumscription of the genus, accepting only Sedum and Pseudosedum within the Sedoideae, dividing the former into 9 sections. Although this was followed by numerous other systems, the most widely accepted infrageneric classification following Berger, was by Ohba (1978). Prior to this most species in Sedoideae were placed in genus Sedum. Of these systems, it was observed "No really satisfactory basis for the division of the family into genera has yet been proposed".
Some other authors have added other series, and combined some of the series into groups, such as sections. In particular Sedum section Sedum is divided into series (see Clades)  More recently, two subgenera have been recognised, Gormania and Sedum.
Gormania: (Britton) Clausen. 110 species from Sempervivum, Aeonium and Leucosedum clades. Europe and North America.
Sedum: 320 species from Acre clade. Temperate and subtropical zones of Northern hemisphere (Asia and the Americas).
Subgenus Sedum has been considered as three geographically distinct, but equal sized sections:
S. sect. Sedum ca. 120 spp. native to Europe, Asia Minor and N. Africa, ranging from N. Africa to central Scandinavia and from Iceland to the Ural Mountains, the Caucasus and Iran.
S. sect. Americana Frod.
S. sect. Asiatica Frod.
S. sect. Sedum includes 54 species native to Europe, which Berger classified into 27 series. Nikulin and colleagues (2016) have recommended that, given the monophyly of Aeonieae and Semperviveae, species of Sedum outside of the tribe Sedeae (all in subgenus Gormania) be removed from the genus and reallocated. However this does not resolve the problem of other genera embedded within Sedum, in Sedeae.
Of about 80 Eurasian species, series Rupestria forms a distinct monophyletic group of about ten taxa, which some authors have considered a separate genus, Petrosedum. It was series 20 in Berger's classification. Native to Europe it has escaped cultivation and become naturalized in North America.
Embedded within the Leucosedum clade are the following genera: Rosularia, Prometheum, Sedella and Dudleya.Rosularia is paraphyletic, and some Sedum species, such as S. sempervivoides Fischer ex M. Bieberstein are assigned by some authors to Rosularia, as R. sempervivoides (Fischer ex M. Bieberstein) Boriss.
Distributed in mainly in temperate to subtropical climates the Northern hemisphere, extending to the Southern hemisphere in Africa and South America, being most diverse in the Mediterranean, Central America, Himalayas, and East Asia. In this respect, the two subgenera differ. Subgenus Sedum having a centre of diversity in Mexico, and Gormania in Eurasia with a secondary centre in N America. 
Many sedums are cultivated as ornamentalgarden plants, due to their interesting and attractive appearance and hardiness. The various species differ in their requirements; some are cold-hardy but do not tolerate heat, some require heat but do not tolerate cold.
The leaves of most stonecrops are edible, excepting Sedum rubrotinctum, although toxicity has also been reported in some other species. The juice from the stems and leaves may irritate skin if handled excessively.
Sedum can be used to provide a roof covering in green roofs, where they are preferred to grasses. Examples include Ford's Dearborn, Michigan Truck Plant, which has a living roof with 454,000 square feet (42,200 m2) of sedum. The Rolls-Royce Motor Cars plant in Goodwood, England, has a 242,000 square feet (22,500 m2) roof complex covered in Sedum, the largest in the United Kingdom. Nintendo of America's roof is covered in some 75,000 square feet (7,000 m2) of Sedum. The Javits Center in New York City is covered with 292,000 square feet (27,100 m2) of Sedum.
Gallo, Lorenzo (24 August 2017a). "Towards a review of the genus Petrosedum (Crassulaceae): Taxonomic and nomenclatural notes on Iberian taxa". Webbia. 72 (2): 207–216. doi:10.1080/00837792.2017.1363978.CS1 maint: ref=harv (link)
Hart, H. 'T; Alpinar, K. (1991). "Biosystematic Studies in Sedum (Crassulaceae) of Turkey. 1. Notes on Four Hitherto Little Known Species Collected in the Western Part of Anatolia". Willdenowia. 21 (1/2): 143–156. ISSN0511-9618. JSTOR3996600.CS1 maint: ref=harv (link)
Hart, H. 't; Jarvis, C. E. (May 1993). "Typification of Linnaeus's names for European species of Sedum subgen. Sedum". Taxon. 42 (2): 399–410. doi:10.2307/1223149. JSTOR1223149.CS1 maint: ref=harv (link)
Nikulin, Vyacheslav Yu.; Gontcharova, Svetlana B.; Stephenson, Ray; Gontcharov, Andrey A. (September 2016). "Phylogenetic relationships between Sedum L. and related genera (Crassulaceae) based on ITS rDNA sequence comparisons". Flora. 224: 218–229. doi:10.1016/j.flora.2016.08.003.
Ohba, Hideaki (March 1977). "The taxonomic status of Sedum telephium and its allied species (Crassulaceae)". The Botanical Magazine Tokyo. 90 (1): 41–56. doi:10.1007/BF02489468.CS1 maint: ref=harv (link)
Ohba, H (1978). "Generic and infrageneric classification of the old world sedoideae crassulaceae". Journal of the Faculty of Science University of Tokyo Section III Botany 12(4): 139-193. 12 (4): 139–193.CS1 maint: ref=harv (link)