**Abstract**

Human body temperature continues to be of interest to researchers with the newest discovery that it had been steadily decreasing since the mid-1800s, thus affecting our definition of normothermia and the cutoff for fever. Further, body temperature constitutes an explicit manifestation of our circadian rhythm, with temperature trough occurring in early morning and peak in the evening in healthy individuals. On the other hand, human illness, as seen among patients ranging from stable hospitalized ones to the critically ill, was shown to correlate with disturbance or even complete disappearance of the body temperature circadian rhythm. Also, night shift work affects the rhythm and had been associated with increased risk for malignancies, gastroenterological and metabolic disorders. Importantly, quantification of the observed rhythm deviations appears to have diagnostic and prognostic value in medicine. This chapter reviews the determinants of body temperature and the body temperature circadian rhythm, discusses the most prominent published research on associations between the rhythm and human disease, and concludes by outlining possible new research avenues and promising clinical applications in the field of chronotherapy.

**Keywords:** circadian rhythm, body temperature, chronotherapy, fever, hypothermia, personalized medicine
