Poplars and aspens often have very wide geographic distributions, large population sizes and are often highly outcrossing. These life history traits should promote the maintenance of abundant levels of genetic variation in trees and early data based on allozyme diversities also confirm these expectations. However, investigation of variation at the nucleotide level has only recently begun in Populus. Surveys of nucleotide polymorphism in Populus have shown relatively high levels of synonymous diversity, between 0.5-1%. Levels of linkage disequilibrium is also relatively low in Populus, although there seems to be large differences between species. The observations of low levels of LD in Populus are so far confined to coding regions and levels of LD non-genic regions is still uncharacterized. Nevertheless, these features suggest that very fine scale mapping is possible in Populus. In principle using recently developed methods in association mapping it should therefore be possible to map quantitative trait variation down to single causal nucleotide changes in Populus.