Antagonistic coevolution between hosts and parasites, the reciprocal evolution of host defence and parasite counter-defence, has far-reaching consequences for evolutionary ecology, agriculture and medicine. It can drive patterns of biodiversity, population dynamics, the evolution of evolvability and parasite virulence. However, the impact of coevolution on these processes is crucially dependent on the specificity of the interaction between host and parasite. That is, do parasite genotypes specialize on host genotypes, or can host and parasites evolve more generalized patterns of resistance and infectivity, respectively? If hosts and parasites specialize on each other, coevolution will be characterized by cycling of genotypes, favouring ongoing coevolution and the maintenance of diversity. By contrast, if generalists evolve, coevolution will be characterised by a directional arms race, resulting in ‘super-parasites’ and the purging of diversity. Host-parasite specificity will be determined by both the genetic bases of the interaction, and the environment. The environment is likely to affect specificity because there are often fitness costs associated with being a generalist, and costs vary under different environmental conditions. The aim of this proposal is to identify the genetic and ecological drivers of host-parasite specificity, and the consequences of this specificity to diversity, virulence, population dynamics and evolvability. The primary approach will be experimental evolution of a well-studied bacteria and phage system; these organisms undergo persistent coevolution in real time under laboratory conditions. Early stages of coevolution are directional; and there are fitness costs associated with the evolution of generalists which vary between environments. We will complement the work by studies of the specificity between bacteria and phage in natural environments and mathematical modeling.