Parasite life history theory predicts that lifetime reproductive success evolves through differential allocation of energy to life history traits constrained by trade-offs. These life history traits govern the characteristics of parasites such as their virulence, transmission and infection phenotypes, so understanding their evolution is a key concern for infectious disease prediction and management. This thesis uses the powerful tool of experimental evolution to gain a fuller understanding of the factors and constraints involved in parasite life history evolution, using bacteriophage Φ2 as a model. I found that the evolution of life history in this phage is sensitive to spatial structure, UV-C exposure and coparasitism with plasmids, and evolution can be mediated by co-evolution with the host. The high levels of variance I observed here suggest that evolution of parasite life history is more complex than a single trajectory towards a predicted optimum, and likely involves some degree of epistasis or pleiotropy with genes elsewhere on the genome. There was some degree of independent evolution of individual life-history traits, indicating that simple direct trade-offs were not in operation. I demonstrated that co-evolution with the host provided additional mutational input, resulting in a greater degree of evolution in co-evolved populations than those evolved to a static host. Furthermore, I note that co-parasitism with phage and plasmid may provide the necessary conditions for plasmid persistence under fluctuating selection for plasmid-encoded traits, and that the efficacy and suitability of phage as therapeutic agents against plasmid-encoded antibiotic resistance is complicated. No direct link between mutation and phenotype could be elucidated in this study, suggesting that evolution in life history is either governed by genes not examined in this thesis, or involves epistasis and pleiotropy with genes elsewhere on the genome. I concluded that it is important to consider the specific ecology of the focal parasite, its host and any co-occuring symbionts in order to make informed predictions of life history evolution, and general predictions may not be achievable.