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Xylariales is an of fungi within the class Sordariomycetes (also known as Pyrenomycetes), subphylum Pezizomycotina, phylum Ascomycota. It is the only order of the subclass Xylariomycetidae.

Xylariales Nannf., Nova Acta Reg. Soc. Sci. Upsal. Ser. IV. 8(2): 66 (1932) is characterised by the possession of well-developed stromata, perithecial ascomata that are mainly globose, superficial or immersed in a stroma, cylindrical asci with amyloid apical apparatus ranging in form from a simple disk to complex ring systems, typically brown ascospores with germ pores or slits, and hyphomycetous holoblastic anamorphs (Table 1.1) (Barr, 1990; Whalley, 1996; Thienhirun & Whalley, 2004). Members of this Order are mostly wood inhabitants but they also occur on animal dung, fruits, leaves, litter, seeds, soil or are associated with insects (Rogers, 1979, 1985, 2000; Whalley, 1985). Their distribution, although cosmopolitan, is very well represented in tropical host plants and many occur as endophytes (Petrini & Petrini, 1985; Davis et al., 2003; Thienhirun & Whalley, 2004).

Nannfeldt (1932) was the first taxonomist to provide a well-defined classification system for taxonomy. He included six families within the Xylariales — the Diatrypaceae, Hypocreaceae, Hyponectriaceae, Lasiosphaeriaceae, Polystigmataceae (Phyllachoraceae) and Xylariaceae, with the last family designated as the type family. Since the work of Nannfeldt (1932), there have been different views regarding the placement of families within the Xylariales, mainly due to emphasis of different morphological and developmental characters (Miller, 1949; Gäumann, 1952). Luttrell (1951) restricted the Xylariales to those taxa with a Xylaria-type centrum and provided a broad definition of the Order. He included six families, the Chaetomiaceae, Clavicipitaceae, Diatrypaceae, Phyllachoraceae, Pyrenulaceae, and Xylariaceae. Later, Wehmeyer (1975) provided a much clearer definition of Xylariaceae which was more in line with current concepts. Unfortunately, delimitation of families within Xylariales was still far from the current classification scheme. Barr (1990), in her Prodomus to nonlichenized pyrenomycetous members of Class Hymenoascomycetes, provided a very broad concept of the Xylariales, accepting 11 families — the Acrospermaceae, Amphisphaeriaceae, Boliniaceae, Clypeosphaeriaceae, Diatrypaceae, Hyponectriaceae, Melogrammataceae, Phyllachoraceae, Thyridiaceae, Trichosphaeriacae and Xylariaceae.

Since the introduction of molecular systematics, phylogenetically unrelated families were gradually removed from Xylariales to their respective Orders. However, there has been little progress for the delimitation of families in the 1990s since none of the molecular studies have focused on Xylariales (Berbee & Taylor, 1992; Rehner & Samuels, 1995; Spatafora et al., 1998). It was not until Kang et al. (1998, 1999a,b,c,d, 2002) and Jeewon (2002), focused on Amphisphaeriaceae and the related families,t he phylogeny of Xylariales has been effectively reviewed. The recent study conducted by Smith et al. (2003) shown that Xylariales consists of seven families: Amphisphaeriaceae, Apiosporaceae, Clypeosphaeriaceae, Diatrypaceae, Graphostomataceae, Hyponectriaceae and Xylariaceae.

The Subclass Xylariomycetidae is a member of the Class Sordariomycetes. Here is the complete "parentage" of Xylariomycetidae:

• Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
  • Kingdom: Fungi T.l. Jahn & F.f. Jahn, 1949 Ex R.t. Moore, 1980
    • Subkingdom: Dikarya D.s. Hibbett Et Al., in D.s. Hibbett Et Al., 2007
      • Phylum: Ascomycota H.c. Bold, 1957 Ex T. Cavalier-Smith, 1998 - Sac Fungi
        • Subphylum: Pezizomycotina O.e. Eriksson & K. Winka, 1997
          • Class: Sordariomycetes O.e. Eriksson & K. Winka, 1997
            • Subclass: Xylariomycetidae O.e. Eriksson & K. Winka, 1997