Perennial grasses belonging to the genus Molinia are widespread in most of Europe and consist of a polyploid complex of closely related taxa with a confusing taxonomy. Based on extensive sampling at 241 localities in Europe, four cytotypes were identified based on chromosome counts and results of flow cytometry: tetraploids (2n = 36), hexaploids (2n = 54), octoploids (2n = 72) and dodecaploids (2n = 108). While tetra- and dodecaploids were commonly recorded, octoploids were less common and only two hexaploid individuals were identified. Previously reported decaploid counts (2n = 90) from central Europe are probably erroneous and refer to 2n = 108. The tetraploid cytotype is distributed throughout Europe and broadly sympatric with other cytotypes. Octo- and dodecaploids were spatially separated with dodecaploids occurring in the western, central and south-central part of Europe and octoploids in the east-central and southeastern part of Europe. All quantitative characters measured (lengths of lemmas, anthers, caryopses and stomata, lengths of the longest hair on the callus and diameter of the culm below the panicle) showed a linear trend across ploidy levels. Tetra-, octo- and dodecaploid cytotypes formed almost non-overlapping groupings in principal component and discriminant analyses of morphological characters. The following taxonomic concept of this complex is proposed: Molinia caerulea (L.) Moench is a predominantly tetraploid taxon incorporating very rarely reported hexaploid and perhaps also diploid plants; higher cytotypes (2n = 8x, 12x) are considered to be M. arundinacea Schrank, consisting of two subspecies: a dodecaploid subspecies occurring in the southern and western part of central Europe and the octoploid Molinia arundinacea subsp. freyi Dančák in east-central and southeastern Europe.