This chapter discusses several different aspects of the energy balance and physiological homeostasis of incubating birds, ranging from systematic, geographical and life history related variation in energy costs of incubation, to thermal considerations for birds on the nest, links between energy expenditure and fitness, and non-energetic costs of incubation. Our review of the literature shows that, across all bird species, this energy costs amounts to 3.4 times the basal metabolic rate (BMR). This is 15% lower than the cost of chick rearing (2.9 × BMR) for all birds, but is roughly equal to chick-rearing costs in those species in which only the female incubates. Energy costs are typically higher in challenging conditions, such as during incubation in harsh climates. This can impair fitness of parents and offspring, but little is understood about the physiological basis for such costs. We highlight and discuss possible mechanisms by which increased energy expenditure in incubating birds might hamper adult survival and, independently, carry over to also affect nestling phenotype and performance. We end by drawing attention to situations where the primary currency for incubation is not energy-based, which we exemplify by a discussion on the water economy of incubation in desert birds.