The 8 different diploid Gossypium genomes vary about three-fold in genome size. DNA renaturation kinetic analyses more than 30 years ago suggested that much of this variation was attributable to the repetitive DNA fractions, and subsequent cloning and sequencing studies have revealed specific DNA elements and families that contribute to this variation. The relationship between physical quantity of DNA and genetic distance (recombination fraction) in a region shows striking variation along individual Gossypium chromosomes, but an appreciable degree of correspondence across subgenomes and species due largely to conserved locations of centromeres. A substantial and growing collection of bacterial artificial chromosome (BAC) libraries for Gossypium species and genotypes provides a platform for studies of local organization of specific genomic regions, and for global physical characterization (which is in progress for several genomes). Of particular importance in planning for the sequencing of members of the Gossypium genus is the nearly two-fold difference in size between the A and D diploid genome types that have contributed to tetraploid cotton, and the finding that repetitive DNA has been transmitted between these two genomes (especially from A to D) in tetraploid cottons. Additional information being assembled, about the diversity among different members of the major repetitive element families, and the degree of inter-genomic exchange following polyploidization, will be important to devising cost-effective sequencing strategies.