Breeding cotton (Gossypium hirsutum L.)for tolerance or resistance to Verticillium wilt (Verticillium dahliae Kleb.) is at best a formidable task. Whether resistance or tolerance exists in current cultivars may be difficult to define. Most cultivars, where applicable, are described as being tolerant and exhibit symptoms yet produce an "acceptable yield". Two major problems have hampered the development of cultivars highly tolerant or resistant to Verticillium wilt. One problem is the difficulty in screening germplasm. In the field, problems with pathogen distribution and environmental interactions compound the interpretation of germplasm performance. Traditional techniques used in screening for Verticillium wilt resistance may circumvent potential resistance mechanisms or may result in an overwhelming challenge to the plant in terms of inoculum density. Another major problem is the lack of germplasm for use as a source of high resistance (perhaps even immunity) or tolerance. Variability in response ranging from complete defoliation and death to delayed symptom expression, however, does exist and is currently exploited in breeding programs addressing this disease problem. The search for more tolerant or resistant germplasm must continue in order to reduce the annual losses attributed to the Verticillium wilt pathogen.