In this talk I will present several theories related to co-evolution between plants and insects. First I will present a model of predator-prey coevolution, showing that rapid evolution in the predator can lead to prey diversification and a decrease in the number of preys available to the predator. This correspond to a "Red King" scenario, where rapid evolution leads to an ecological disadvantage. Second I will present the evolution from an antagonistic to a mutualistic interaction. Finally I will contrast expected patterns of coevolution in mutualistic and antagonistic networks.