Time-lapse video techniques were used to study the responses of polyphagous invertebrate predators to aggregations of aphids and alternative prey in the laboratory and field. The aim was mainly to rank the predators in terms of their aphid control potential. The capture efficiency of pitfall traps in a laboratory arena for seven carabid species was examined. A significant heterogeneity of capture efficiency between the species was obtained. Statements derived from pitfall trapping concerning epigeal fauna are likely to be heavily biased. Data such as those obtained in this thesis could be used to 'correct' field catches. The main group of carabids studied in this thesis was found not to climb cereal plants, in the laboratory and field, in order to obtain their food. This fact, however, was not considered to preclude the use of carabids as potential aphid-control agents because a high number of aphids fall off their host plants each day. In the absence of predation these would return to the crop. The responses of four carabid species to patches of different densities of aphids on the ground in an arena in the laboratory were studied. All of the species (Agonum dorsale (Pont.), Harpalus rufipes (Degeer), Nebria brevicollis (F.) and Pterostichus madidus (F.)) reduced their speed of movement (orthokinesis) and increased their angle of turn (klinokinesis) after having fed on an aphid. A trend demonstrating the aggregative numerical response was observed for all of the species. That is, as the density of aphids in the prey grid increased, the mean time spent walking per individual in this area became significantly higher (above a certain lower density threshold) than in the control grid. An aggregative numerical response to aphid patches on the ground in a wheat field was observed. The same mechanisms leading to the response as observed in the laboratory were apparent. The two carabids, A. dorsale and Bembidion lampros (Herbst), and the staphylinid, Tachyporus hypnorum (F.), were the species most frequently observed. Prey-choice experiments demonstrated that A. dorsale showed a 'preference' for aphids over Diptera and Collembola alternative prey. This was not the case, however, for P. madidus which 'preferred' Diptera to the aphids. The basis of the preferences was discussed. Prey choice experiments with live and dead aphids showed that the small diurnal carabid species 'preferred' live prey, whereas the larger nocturnal species preferred dead prey. In terms of ranking, then A. dorsale would appear to be the most effective aphid predator, in terms of patch responding ability, of the species studied. The implications of using only few aspects of species' ecology for the purpose of ranking were discussed.