On the basis of extended DNA sequences (nrITS, cptrnL-F) and other new evidence present knowledge on the differentiation of the northern hemisphere Compositae-Anthemideae genus Achillea in time and space is surveyed. Evidence is presented for a wide circumscription of the genus with 110–140 species. This includes the former unispecific genera Otanthus and Leucocyclus, which have originated from ancestors of the basalmost clade of Achillea, the xerophytic A. sect. Babounya s.l. (incl. sect. Arthrolepis and Santolinoideae). Maps show the predominantly SW Asian distribution of the section and its derivative, A. (sect. Otanthus) maritima, a psammophyte of the Mediterranean and Atlantic coasts. Through taxa from the Caucasus differentiation has proceeded from A. sect. Babounya-like ancestors to the Eurasian and more meso- and hygrophytic 2x members of A. sect. Ptarmica s.s. They are linked with 2x A. asiatica (A. millefolium agg.) by the hybrid 4x A. alpina agg., which has expanded across northern North America. A. sect. Anthemoideae s.l. consists of several distinct clades distributed in the mountain systems of NW Anatolia, the Balkans, Carpathians, Alps, Apennines and E Pyrenees. This section was formerly combined with A. sect. Ptarmica but is closer to the following section and may be polyphyletic. A. sect. Achillea includes closely related taxa with white and yellow(ish) ligulate flowers (the latter formerly separated as A. sect. Filipendulinae). Members of the section have differentiated from a centre of diversity in dry habitats of SE Europe, partly into uniform 2x species, partly into polymorphic and polyploid species groups. “Crown group” of the genus and section is A. millefolium agg., a strongly diversified polyploid complex. Its basal 2x species exhibit a vicarious and sometimes relic differentiation pattern. They are connected in a complex way to several parallel and reticulate lines of 4x, 6x and 8x taxa, which are partly very expansive. From 4x A. asiatica-like ancestors in E Siberia the 4x and 6x A. borealis has successfully radiated into many ecological niches in North America and 6x A. millefolium s.s. has become a cosmopolitan weed.