It has been suggested that parasitic interactions in general result in co-evolutionary arms races where parasites and hosts evolve adaptive behaviours and traits to maximize their fitness in a conflicting manner. Avian brood parasitism provides an ideal system for the study of such co-evolutionary interactions. In this paper I show and discuss possible consequences of the co-evolutionary arms race inherent in avian brood parasitism, especially focusing on egg appearance, a key factor for the reproduction of both hosts and parasites. In such an arms race, hosts evolve the ability to recognize and hence to reject dissimilar parasitic eggs, while parasites counter host defences by more closely mimicking their eggs. Egg mimicry by parasites might then be de-stabilized by hosts changing the appearance of their eggs, thus exposing the mimics. The evolutionary trajectory of the arms race varies depending on the specific detailed mode of interaction between parasite and host individuals. Mathematical modeling, as an important parallel approach to empirical study, helps point the way for further study of avian brood parasitism so as to better understand co-evolution in general.