Life-history theory also has a broad range of practical applications, from being a key tool for estimating maximum sustainable yield in commercial fisheries, to explaining why girls are reaching maturity at an increasingly earlier age. As in many species, age at maturity in Atlantic salmon is tightly linked with size at maturity representing a classic life-history trade-off: later maturing individuals spend more time at sea before returning to freshwater to spawn and have higher reproductive success due to their larger size but also have a higher risk of mortality prior to first reproduction. Our research capitalizes on the unique opportunities provided by the simple genetic architecture of salmon age at maturity as an important life-history trait. We use Atlantic salmon as a model for integrative functional genomic, ecological and evolutionary research. We conduct functional analyses in controlled conditions for understanding cellular processes. Our research in semi-natural environments and managed populations enables us to better understand GxE effects, and how selection operates on these processes under more realistic environmental conditions. Finally, research on undisturbed populations, capitalizing on the predictability and philopatry of salmon spawning, enables us to estimate reproductive fitness that can be linked to maturation genotype and age at maturity phenotype and thus determine the relative importance of natural and sexual selection in maintaining diversity in age at maturity within and among populations. We also apply the knowledge we obtain for improving conservation and management of this iconic species.