The role of sperm age-related effects on male and female fitness has been underappreciated in evolutionary biology, although it may explain a significant portion of the unexplained variation in the reproductive success of both sexes. These effects are thought to be twofold. First, eggs fertilized by aged sperm with impaired structure and/or damaged genetic material may suffer lower fertilization prospects and possible transgenerational effect that compromise offspring survival and fitness. Such effects may generate sexual conflict and explain why female typically mate multiply (in species where direct benefits of polyandry are not obvious). Second, any cell damage incurred through sperm ageing is likely to impair sperm performance, leading to a decline in male reproductive success due to a reduction in sperm competitive ability. This is likely to result in potentially dramatic fitness effects in males in species with high levels of sperm competition. In the present study we are using the guppy (Poecilia reticulata) to explore the implications of sperm senescence for both male and female fitness. We first assess the effect of sperm senescence on sperm performance traits such as velocity and viability before exploring the fitness implications of sperm ageing by assessing the relative fertilization success of aged and fresh sperm when they compete for fertilization following artificial insemination. Finally we assess the effects of sperm ageing on female fecundity and components of offspring fitness, including body size at birth and after sexual maturity, survival and a range of sexually selected traits. Our analyses will assess importance of considering sperm ageing within an evolutionary framework.