This review reevaluates the importance of interspecific competition in the population biology of phytophagous insects and assesses factors that mediate competition. An examination of 193 pair-wise species interactions, repre senting all major feeding guilds, provided information on the occurrence, frequency, symmetry, consequences, and mechanisms of competition. Inter specific competition occurred in 76% of interactions, was often asymmetric, and was frequent in most guilds (sap feeders, wood and stem borers, seed and fruit feeders) except free-living mandibulate folivores. Phytophagous insects were more likely to compete if they were closely related, introduced, sessile, aggr egative, fed on discrete resources, and fed on forbs or grasses. Interference 297