In this paper, we reviewed scientific literature on hair cortisol as an indicator of chronic stress. Cortisol, which is the end product of the hypothalamus-pituitary-adrenal (HPA) axis, is considered a reliable indicator of chronic stress level. Existing literature then suggests that, when compared to other biomarker such as saliva, urine, and blood, hair appears more feasible and reliable. For this reason, it has gradually become the preferred method for assessing chronic stress level. Collective evidence indicated that hair cortisol is accurate in (1) confirming that certain life events caused chronic stress; (2) revealing that particular groups of people are vulnerable to chronic stress; and (3) reflecting that an individual suffers from certain psychological as well as physical difficulties. We also discussed future directions for this line of research. We advocate for developing more accurate psychological tests for assessing chronic stress, and for wider application of hair cortisol as a biomarker in pertinent research contexts such as intervention studies as well as longitudinal studies.