||Because plants and their insect enemies are strikingly species-rich groups, understanding their interactions has been a key issue in ecology and evolution. The arms race between plants and herbivores is considered the driver of diversification in both groups. However, we have a poor understanding of how these processes lead to divergence and speciation. This dissertation research tests key theories that relate plant- insect interactions with diversification and coevolution in both groups of organisms. In the first part, I assess the utility and contrast the predictions of two theories aimed to explain the patterns of defense investment across species: The Apparency Theory and the Resource Availability Hypothesis. My results provide strong support for the predictions of the Resource Availability Hypothesis. In particular, the evolution of defenses appears to be related to interspecific differences in inherent growth rate rather than to a species' predictability to herbivores. The theory appears robust across latitude and ontogeny suggesting that it has served as a valid framework for investigating the patterns of plant defenses and that its applicability is quite general. In the second part, I focus on how herbivores may drive the evolution of plant defenses, how plant defenses shape herbivore host choice and how plant-herbivore interactions might influence community composition and diversity focusing on the Neotropical genus of trees Inga (Fabaceae). I characterize the entire suite of anti-herbivore defenses and also quantify the diversity and abundance of leaf-feeders associated with Inga. With the use of phylogenies for both plants and herbivores, I discriminate among possible macroevolutionary hypothesis of host use and plant defense evolution. Contrary to much coevolutionary theory, my results show that closely related Inga species are more divergent in anti-herbivore defenses than in non-defense traits, and that the evolution of host use in herbivorous insects is more conserved with respect to host defenses rather than to host phylogeny. Together, these results suggest that defenses evolve rapidly and that traits related to host choice evolve more slowly. Specifically, although divergence in herbivores might not be driven by their interactions with plants,herbivores may be an important factor driving the divergence among plant species.