The aim of the project ‘The genetics of robustness in laying hens’ was to investigate nature and regulation of robustness in laying hens under sub-optimal conditions and the possibility to increase robustness by using animal breeding without loss of production. At the start of the project, a robust animal was defined as ‘an animal under a normal physical condition that has the potential to keep functioning and take short periods to recover under varying environmental conditions’. Next, parameters or traits were selected that could give an indication of robustness, and that could be implemented into a breeding goal for robustness. The experiments described in this thesis investigated parameters of interest for robustness in laying hens, where the influence of genetic background, environmental conditions, and early-life experiences was used as framework. The first experiment aimed at genetic differences in innate (natural) humoral immune components between 12 purebred layer Lines. Levels of innate and specific immune competence depended on genotype. Within layer lines, however, innate immune competence was related with survival. The second experiment aimed at influence of, or response to, environmental conditions, i.e., climatic stress (high temperature) and microbial challenge (lipopolysaccharide). Comparable response patterns to climatic stress and microbial challenge were found within lines, but lines differed in response levels towards these stressors. The third experiment aimed at improvement of robustness by early-life experiences to climatic stress and microbial challenge. The data did not reveal improvement of robustness by early-life experiences. Results from these studies indicate that robustness mainly depends on genetic background and environmental circumstances and to a minor extent on early-life experiences. The basic elements for robustness are survival, reproduction, and responsiveness towards environmental stressors, where a robust laying hen is a hen with a high survival rate, high production level, and low responsiveness towards environmental stressors. We have established a predictive value for the level of NAb binding to KLH for survival of the laying period of laying hens. Besides, NAb have a moderate heritability, giving opportunity for selection towards this trait. Performance parameters and innate immune parameters are most likely not related and selection on innate immune parameters will probably not be on the expense of hen-day egg production. Implementation of selection for NAb into a breeding goal might, therefore, improve robustness of laying hens.