Aphids are economically important pests that predominantly feed from the plant phloem. Genome, transcriptome, and proteome data are being generated for these insects, and predicted secreted proteins in aphid saliva have been identified. These secreted proteins are candidate effectors that may modulate plant processes and aid aphid colonization of plants. The next step is to develop post-genomics strategies to study the functions of identified aphid genes. One such strategy is to express aphid effector genes in planta to assess whether aphid effectors alter plant development and aphid survival and fecundity. A second strategy is to knock down the expression of aphid target genes by RNA interference (RNAi). In this chapter, we describe how to knock down aphid gene expression using plant-mediated RNAi. This strategy is useful for assessing the contribution of aphid effectors to aphid colonization of plants.