Cortisol, a neuroendocrine hormone measurable in saliva, responds to internal and external triggers. In providing a peripheral ‘window on the brain’, it has been increasingly incorporated into social psychological studies. Cortisol secretion can be studied in two main ways, examination of acute stress reactivity and examination of the basal circadian patterns. These can inform aspects of acute and chronic stress exposure and relationships with health. Within non-clinical populations, cortisol effects are largely driven by differences in the perception of threat. For social psychologists, this provides an interesting avenue for the investigation of social factors that mediate perceptions of threat, such as social support, relationship processes, and group dynamics in acute and chronic stress. This paper provides a background to understanding the regulation and function of cortisol, and issues arising in relation to its measurement in saliva. It discusses and makes recommendations on the use of appropriate cortisol measures in the study of both acute and chronic stress. Used and interpreted appropriately, stress reactivity and basal ambulatory measures of salivary cortisol can provide a valuable adjunct to self-report and observation in social psychological research.