In this review we have described, in some detail, the physical processes involved in water loss from both the skin and lungs. Although at first glance these physical processes may seem complex and confusing, once the basic concepts are grasped, the effect of the many variables in both the babies and their environment on IWL can be seen more clearly. Measurement of IWL, or its components, TEWL and RWL, is difficult in newborn infants. Some of the difficulties arise because of the nature of the subject being studied, and because of inaccuracy in the measuring apparatus. The difficulties in the subjects include lack of cooperation, and the presence of severe illness, both of which may limit the representativeness of any sample of babies that is eventually studied successfully. The size of the subjects studied means that small amounts of water are given off in any fixed period of time. Consequently, the accuracy of the measuring instruments has to be high. As we have discussed, each of the methods used to estimate IWL, TEWL, or RWL has limitations and potential inaccuracies. Despite the difficulties in obtaining estimates of IWL in newborn infants, there have been many studies over the years that have provided clinically useful data. More recently, improved survival of VLBW infants has lead to an awareness that IWL is substantially increased in these tiny babies. The best way to manage the problems of water and heat balance associated with increased IWL in VLBW infants remains to be determined.