Energy balance, simply, is the difference between energy consumed and sum of energy used for maintenance and production. The negative difference indicates body reserves being mobilized, whereas the positive difference indicates body reserves being replenished or increased. Negative energy balance (NEB) is accepted as natural event in dairy cattle because, dry matter intake decrase during the close-up period and increases more slowly than milk yield after parturition. Other than phenotypical changes (body weight and condition score change), metabolic indicators in blood and milk (nonesterified fatty acids, ketone bodies, milk fatty acid composition, and milk fat:protein ratio, etc.) also change, which can be used for estimating severity and duration of NEB. This review emphasizes causes of NEB, metabolic indicators that may be used to estimate NEB, and important points in their interpretations.