The tristeza disease of citrus has not yet been reported from European countries nor from various other citrus growing areas of the world. Investigations on the disease have been carried out in various countries, such as Argentina, Australia, Brazil, South Africa, and United States. Knowledge obtained from these investigations enables citrus experts from areas not invaded by tristeza to recognize its presence in the orchards and to prevent disastrous losses for the industry. General decline symptoms of tristeza resemble those caused by foot rot, but root examination permits the differentiation of the two diseases. Presence of pitting symptoms in trees of West Indian limes or of grapefruits permits easy diagnosis of tristeza in the orchards. Stock-scion combinations may be tolerant or intolerant to tristeza. Thus, it be comes important to determine the rootstock in case of plants showing decline symptoms. This fact can be established by direct examination of sprouts from the rootstock or by observing the degree of congeniality of rootstock and scion. The colorimetric tests made with bark of the rootstock may help to determine it. Field observation of symptoms complemented by Schneider's histological tests may replace the transmission tests for field surveys of the disease in areas where tristeza is already known to be present. Transmission tests are usually carried out by methods of tissue union or by means of the vectors. Young plants of non-tolerant combinations (such as sweet orange on sour orange rootstock) or seedlings of West Indian limes may be used as test plants in these tests. A knowledge of the predominance of the various strains of the tristeza virus complex in the invaded areas is very important in regard to the application of control measures. Also the determination of the aphid vector or vectors in the areas concerned will permit forecasting the rate of spread of tristeza because efficiency varies considerably according to the vector species. Control of tristeza in citrus orchards is obtained by indirect methods. In new plantings made in invaded areas, non-tolerant stock-scion combinations should be avoided. Only tolerant rootstocks such as sweet oranges, mandarins, rangpur lime, and rough lemons should be used for most commercial tops. In some instances it may be possible to use unbudded trees originated from nucellar embryos. In established orchards, where a non-tolerant rootstock was employed, individual affected trees may be replaced or the whole planting may be interplanted with trees of a tolerant stock-scion combination. The first method is used in areas where the disease is disseminated by a poor vector, whereas the second method is more advisable in areas where spread of the disease is very rapid. Inarching or rooting the scion (by mounding the soil around its base) are used to control tristeza under some conditions. Top working with lemon may also be used. Quarantine barriers may prevent the introduction of tristeza in uninvaded areas. It is highly desirable to inform travellers about the risk represented by the introduction of citrus budwood material or cuttings from areas where the disease is known to be present.