The study summarised in this chapter tested a set of hypotheses concerning the drivers of ploidy state in aquatic macrophyte communities at worldwide scale, and also how ploidy state may be related to other attributes of aquatic plants such as community diversity, species range size, and endemism. The chapter is based upon a global ploidy database with 1574 species (representing some 45.0% of the known total global macrophyte species pool), and examines the association of macrophyte assemblage ploidy state with 16 global scale spatial, landscape, environmental and biotic variables. The results demonstrate that (1) polyploid and mixed ploidy species (having both haploid/diploid and polyploid races) occupied larger geographical ranges than haploid/diploid species; (2) there were contrasting latitudinal patterns between haploid/diploid and polyploid species, while species richness measures also influenced ploidy state; and (3) climatic factors, especially temperature and evapotranspiration, are congruent with macrophyte species ploidy state, across the planet. These findings support the assertion that increased chromosome number is an adaptation to plant survival in harsh environmental conditions, especially in environmentally stressed high latitudes. Our chapter increases the generality of knowledge of the large-scale factors that drive global cytotype patterns in vascular plant species.