World aphid species attacking citrus are reviewed. The 4 aphids, Toxoptera citricidus, T. aurantii, Aphis gossypii and A. spiraecola are the major species whereas other 11 species are less important. Their occurrence varies with the countries or districts. Aphid life cycle in general and that of the major 4 species are described. Ecology of and factors affecting the occurrence of 3 important citrus aphids in Japan are described and relations between aphid populations and some factors controlling aphid populations are outlined. Transmission of citrus tristeza virus (CTV) is reviewed and transmission rate of T. citricidus and A. gossypii is compared in relation to different strains of CTV or of aphids. All of 4 world important species can transmit CTV. Especially, T. citricidus and A. gossypii are efficient vectors of CTV in different areas of the world.