Over the past 15 years, the population of sick neonates has changed. The average birthweight has decreased dramatically to as low as 450g while gestational age has been reduced to 22-21 weeks. These babies have totally different needs and require more complex health care. It is now time to change previous recommendations which were based on more mature neonates. In this foreword we aim to give some of the background into the ideas which lead to us starting work on new guidelines. The question was: Can the thermal condition of the baby be better monitored and investigated and, if so, how? Measurement of body temperature remains a very important method of ascertaining the thermal condition of a newborn baby. There is an implicit international agreement that the body temperature of an infant is measured either intermittently using a liquid expansion thermometer or continuously by means of temperature sensors placed in the rectum or the axilla. Rectal temperature is not necessarily identical to the core temperature. There may be up to a 2 C difference and this could result in an incorrect assessment of the condition of the patient. For example in the presence of cold lower limbs, the cool venous blood returning from the legs will flow close to the rectum and, as a result, the rectal temperature may be significantly lower than that of the core. Other workers have shown how rectal temperature depends on the depth to which the sensor has been inserted.