The biology of stress in chickens is reviewed. Not only is stress associated with depressed production, but animal welfare influences consumer acceptance of poultry and eggs. The reciprocal of well-being is stress. The hypothalamo-pituitary-adrenocortical axis in poultry consists of the neuropeptides, corticotropin releasing hormone, and arginine vasotocin that are released from the median eminence; the polypeptide hormone, adrenocorticotropic hormone (ACTH) secreted by the anterior pituitary gland; and the glucocorticoid hormone, corticosterone (CORT), synthesized by the adrenocortical cells. Many, but not all, stresses in chickens increase circulating concentrations of CORT. Circulating concentrations levels of CORT (both basal and in response to stressors) show marked differences in the literature, suggesting further attention is needed to ensure assays are validated for CORT in chicken plasma and other sources - excreta and feathers. As glucocorticoids influence the heterophil:lymphocyte (H:L) ratio, it is not surprising that the H:L is shifted with stress. It is recommended that close attention needs to be placed on the validity of assays including cross-laboratory standards. In addition, there is a strong case for determining multiple parameters of stress.