Technical Abstract: Only a very small fraction of the genetic diversity residing in the Gossypium genus is represented in improved, elite cotton germplasm. Although genetic diversity in elite germplasm is reported to be narrow, diversity on the farm is narrower, due to preferential planting of successful cultivars, and breeding techniques that tend to promote an over-reliance on a few genotypes. By contrast, the genetic variability present in in situ populations and ex situ collections represents a large, under-utilized resource for cotton improvement efforts. In situ resources are in many places being threatened by human development or are disappearing. Ex situ resources have been reported to include 49,000 accessions residing in over 25 institutional germplasm collections worldwide. In the United States, the Agricultural Research Service (ARS) of the USDA maintains a Gossypium collection of 9,303 accessions, representing 42 species. These accessions are divided into primary, secondary, and tertiary germplasm pools according to their relative genetic accessibility and utility. In 2006, 4,106 accessions of the USDA-ARS collection were distributed to 105 researchers and institutions, worldwide. Among problems challenging the ARS collection are limited resources for seed renewal, incomplete characterization and evaluation of accessions, and unrecognized gaps and redundancies in the collection. Efforts to find and direct resources toward these problems are ongoing. Outside the USDA-ARS germplasm collection, numerous genetic resources for the investigation and improvement of cotton have been created by public researchers, including recombinant inbred line (RIL) populations, back-cross introgression line (BIL) populations, near isogenic introgression line (NIIL) populations, chromosome substitution lines, and day-neutral converted race stocks, among others.