Cereal aphids are important as direct pests of crops and as virus vectors. Several species, including the corn-leaf aphid (Rhopalosiphum maidis) and the green bug (Schizaphis graminum), persist and reproduce parthenogenetically throughout the year in the southern areas of the United States. They persist less readily or not at all in the northern states and Canada where winters are cold and these areas are reinvaded annually by migrants from the south, some of which are likely to be viruliferous. This paper reviews studies on the reinvasion phenomenon including the recent `Pests and Weather' project in Illinois (1983-1985) in which radar and traps mounted on a helicopter were used to detect cereal aphids in flight at altitudes of up to 1100 m. A new back-tracking procedure, electrophoretic analyses and fuel utilization studies were used to determine possible sources of the aphids caught. The studies on cereal aphid vectors in North America and the various difficulties encountered are discussed in a wider geographic context and in relation to studies on other types of vectors. It is concluded that long-range dispersal has received inadequate attention in relation to its biological and economic importance.