A taxonomic revision of the highly diverse Western Australian Mulga flora (Acacia aneura F.Muell. ex Benth. and its close relatives) is presented, based on morphological analyses of almost 2000 herbarium collections complemented by field studies of about 300 populations (mostly located in Western Australia, with a few from the Northern Territory and South Australia). Twelve species accommodated in three informal groups, the Blue, Grey-green and Green Alliances, are recognised. Almost all of the species contain informal variants, and putative hybrids and/or intergrades are common. Because many of the species had previously been recognised as varieties of A. aneura (Pedley 2001) a discussion of our taxon concepts is given. The 12 species are defined by a combination of morphological characters, the most important being branchlet resin (translucent vs opaque) and pod margins (rimmed, bevel-edged or winged), complemented by new shoot (resinous vs non-resinous), phyllode (shape, size, curvature, nervature) and pod (width) attributes. A discussion of the taxonomically most informative characters in the Mulga group is presented. Seven new species are recognised: Acacia aptaneura Maslin & J.E.Reid (syn. A. aneura var. pilbarana Pedley and A. aneura var. tenuis Pedley), A. caesaneura Maslin & J.E.Reid (syn. A. aneura var. argentea Pedley), A. fuscaneura Maslin & J.E.Reid (syn. A. aneura var. fuliginea Pedley), A. incurvaneura Maslin & J.E.Reid (syn. A. aneura var. microcarpa Pedley), A. macraneura Maslin & J.E.Reid (syn. A. aneura var. macrocarpa Randell), A. mulganeura Maslin & J.E.Reid and A. pteraneura Maslin & J.E.Reid. Acacia aneura var. intermedia Pedley is provisionally regarded as conspecific with the broadly circumscribed A. aneura.