Dragonflies and damselflies (Odonata) are important prey for many small falcons, and the recent expansion of the breeding range of the Hobby into eastern England has been associated with an increased availability of dragonfly prey to juveniles. We have therefore measured the energy content and elemental composition of a range of common British dragonflies. Carbon and nitrogen contents were typical of aquatic invertebrates and did not vary with dragonfly size, whereas ash content was significantly reduced in larger species. The mean energy content of dragonfly tissue was 24.6 kJ/g (dry mass) and showed no significant variation between species. The energy content of an individual dragonfly ranged from 0.8 to 9.4 kJ for the species examined in this study. Data on the energy requirements of free-living falcons suggest that a juvenile Hobby in late summer could meet its daily energy requirement by capturing between 75 and 90 Migrant Hawker or 200 to 250 Common Darter dragonflies each day.