This thesis is an investigation of the effects of maternal stress on the offspring of different genetic lines of layer breeders. In their lifetimes, layer breeders may experience stressors that can lead to increased levels of corticosterone or other components in the egg, potentially impacting their progeny (commercial laying hens). Thus, the main objective of this thesis was to investigate whether some layer breeders of different genetic background are more sensitive to the effects of maternal stress than others. Additionally, this study aimed to assess if the glucocorticoid hormone corticosterone mediates the effects of maternal stress. For this, two stress models were tested in five strains of laying hens (4 commercial lines and one pure line): A maternal stress model, in which layer breeders from each genetic line were subjected to a series of daily psychological stressors during egg production, and a pharmacological stress model, in which a sub-sample of eggs from non-stressed breeders received one single injection of corticosterone moments before incubation. Measures of hatchability, body weight, fearfulness and stress response were assessed in the offspring. The pharmacological stress model decreased the hatchability and body weight of the progeny from hatch to 17 weeks of age, while the maternal stress model decreased response to social isolation (measured by the total number of distress calls) but only in one commercial line without further effects in any other traits measured. These results suggest that layer breeders may be more resilient to the transfer of maternal stress than initially predicted. Moreover, it suggests that corticosterone might be involved in the transfer of maternal stress in laying hens, but not as the primary mediator. Regardless of stress treatments, behavioural and physiological differences across different strains of layers were consistently observed, especially between brown and white strains, highlighting the individual characteristics of each genetic line. In conclusion, this research helped clarify the effects of genetic variation among commercial and pure genetic lines of layer breeders in the field of maternal stress.