The relevance of measuring core temperature for diagnosis and treatment of various diseases is generally acknowledged nowadays. Despite introduction of new techniques and markedly improved understanding of body temperature regulation and the pathophysiology of fever, several dilemmas remain to be elucidated in clinical thermometry. In the measurement and interpretation of body temperature, the many variables that influence core temperature, the site of temperature registration and the significance of an elevated or decreased core temperature should be taken into account. With every type of thermometer, good calibration and an adequate recording technique remain pivotal to obtain reliable and reproducible results.