This monograph investigates evolution of resistance against an intracellular microsporidian parasite Tubulinosema kingi and the associated trade-offs in Drosophila melanogaster model system. Additionally, it also investigates the interactions between the host and its parasite at a physiological level. The implications of these results are further discussed in relation to other host-parasite systems and the possibility of using the system as a model insect- microsporidia system to explore ecological and evolutionary interactions in greater depth.