Nutrition is an important driver of biological variation. Macronutrients such as protein and carbohydrates, and elemental nutrients such as phosphorus, are known to affect animal fitness traits. No study, however, has investigated the importance of phosphorus relative to dietary protein or carbohydrates, or their interactive effects, on animal performance. To advance our understanding of the impact of nutrition on individual fitness, my thesis examined the influence of dietary protein, carbohydrate, and phosphorus balance on fitness-related life-history traits, including those involved in intra- and inter-sexual selection, of Gryllus veletis field crickets. My findings revealed that adult lifespan, weight gain, males' acoustic mate attraction signals, and females' egg production were maximized on diets with different protein:carbohydrate ratios, such that not all fitness traits could simultaneously be maximized on the same diet. Similarly, juvenile females could not simultaneously maximize their growth, development rate, survival, and dispersal capability at adulthood on the same dietary protein:carbohydrate ratio. Adult males and females also had different optimal nutrient intake ratios for reproductive performance. My results support theoretical predictions for the condition-dependence of traits involved in inter- and intra-sexual selection; both male mate attraction signals, and female sexual responsiveness and preferences for such signals, were influenced by dietary protein:carbohydrate ratio. However, male aggressiveness in agonistic contests with rivals was not influenced by dietary nutrient balance. Contrary to my expectations, dietary phosphorus had little influence on fitness traits, with the exception of a negative influence of high phosphorus diets on male mate attraction signals. When given a choice between diets containing differing but complementary nutrient compositions, both adults and juveniles demonstrated an ability to regulate their protein and carbohydrate intakes, but not their phosphorus intake. These self-selected diets often represented a compromise between the differing nutrient requirements of multiple fitness traits. Overall, my findings suggest that environmental heterogeneity in nutrient availability is an important driver of variation in animal fitness, and highlight the importance of disentangling the influences of different nutrients on animal fitness traits.