Seabirds share certain life history traits, most species being relatively longlived and reproducing at a low rate. Long-term population studies of seabirds have revealed that lifetime reproductive success differs widely among individuals, and that only a minor fraction of the population contributes importantly to future generations. These individuals have traditionally been termed of high parental quality. Quality is often defined tautologically as breeding success. Determinants of parental quality, and ways to measure it a priori without relying on breeding success, remain a challenge in seabird ecology. Parental health state and immunocompetence have been shown to be associated with breeding date, egg size, chick growth rate and breeding success in several field studies and may allow the identification of good breeders. Identifying the high quality fraction in a population may be worthwhile in order to better guarantee the productivity of declining populations.