Recent developments in the theory of ecological speciation and adaptive radiation emphasize the importance of ecology in the process of evolutionary diversification. For parasites, such as host plant-specific herbivorous and parasitic insects, host-associated differentiation of populations is a major mechanism by which ecological specialization might lead to evolutionary diversification. However, most conceptions of the process of host-associated differentiation (HAD) have been limited to a bi-trophic perspective. That is, it has been assumed that the interaction between the parasite and the host plant solely determines the process of differentiation. We present HAD in the context of multi-trophic interactions. We believe complex interactions between plants, herbivores, and carnivores may be generally important for understanding HAD. A growing number of studies has shown, for example, that variation in herbivore performance and fitness on particular host plants is highly dependent on interactions with natural enemies. Likewise, carnivores such as parasitoids depend on plant-herbivore interactions for critical processes such as host location. Recent studies provide empirical evidence that morphologically cryptic HAD is more frequent than previously thought in plant-feeding and parasitic insects. HAD has been widely demonstrated in herbivorous insects at the phenotypic as well as at the genetic level. In the sympatric Solidago altissima and Solidago gigantea system, HAD has been shown to occur in a diversity of herbivorous and carnivorous taxa, suggesting HAD may be an important generator of insect diversity. Speakers in this session will discuss how multi-trophic interactions can provide insight into HAD. How common is HAD in nature and where is it likely to be found? Is HAD ubiquitous (as the literature on insect herbivores suggests) or is it restricted by requirements that are more rarely fulfilled? Do plant characteristics such as chemistry, architecture, or life history determine the likelihood of HAD at the second or third trophic level? Can morphologically cryptic HAD in predators and parasitoids be important in determining the success or failure of biocontrol programs?