Diversity of ants of the Tetramorium caespitum/impurum complex was investigated in a multidisciplinary study. Focusing on morphologically hardly distinguishable Western Palearctic samples, we demonstrate the genetic and phenotypic diversity, demarcate phylogenetic entities, and discuss the clades in terms of biogeography. Sequences of 1113bp of the mitochondrial COI gene revealed 13 lineages. COII data, worker morphometry and male genitalia morphology corroborated the COI results for seven lineages; the remaining six were disregarded because of small sample size. A comparison with published data on cuticular hydrocarbons showed correspondence. The seven entities show different distribution patterns, though some ranges overlap in Central Europe. Since no major discrepancy between the results of the different disciplines became apparent, we conclude that the seven entities within the T. caespitum/impurum complex represent seven species. Geographical evidence allows the identification of T. caespitum and T. impurum, and we therefore designate neotypes and redescribe the two species in terms of morphology and mtDNA. As the revision of about 50 taxon names would go beyond the scope of this study, we refer to the remaining five species under code names. We discuss our findings in terms of plesiomorphy and convergent evolution by visualizing the mtDNA phylogeny in morphological space.