Hormones mediate individual decisions on behavior and life history strategies depending on internal and external conditions. The glucocorticoid hormone corticosterone (CORT) is involved in metabolic processes and thus facilitates adjustments in behavior and life history strategies to energetic circumstances. Vertebrate populations can show substantial within- and among-individual variation in circulating CORT concentrations, raising questions on the causes of such variation and its phenotypic consequences. Furthermore, to understand evolutionary processes we need to determine whether CORT traits of individuals are repeatable and heritable, whether they are related to fitness components and which selection pressures they may underlie. To approach these questions, we assessed relationships among CORT traits, behavior, internal and environmental challenges and fitness in individual great tits (Parus major), combining field and captive studies. In great tits, circulating CORT concentrations were related to behavioral phenotypes including exploratory behavior, parental investment, and pair bond dynamics. Circulating baseline CORT concentrations were also related to reproductive success in wild great tits. Even though CORT concentrations showed considerable within- and among-individual variation, we found significant repeatabilities for CORT traits, especially when environmental variation was reduced in captivity. Given the high sensitivity of CORT concentrations to both internal and external cues, we have begun to test whether individuals exhibit consistent reaction norms in CORT traits, which may constitute an important component of an individual’s hormonal organization. Our work aims at elucidating evolutionary patterns in hormonal phenotypes of wild avian populations.