This study examines the kinematics, aerodynamics, and energetics of forward flight in insects. Bumblebees and hoverflies in controlled free flight were filmed over a range of flight speeds with a high speed camera at 5000 frames a second. The wing motions in three dimensions were reconstructed using a projection analysis technique. A new method was developed to determine from a single film image the span-wise angle of attack of the wing. The variation with forward speed of kinematic parameters describing the nature of the wing beat is described. The results of detailed morphological analyses of the wings and bodies of those insects filmed in free flight are presented. Measurements of lift and drag on isolated insect wings and bodies were made with an optoelectronic force transducer in order to determine their steady-state aerodynamic characteristics. The variation of lift and drag coefficients with airspeed and object orientation was determined for use in later calculations of lift and power requirements. Mean lift and drag coefficients, as averaged spatially for the whole wing and temporally for the wing-beat, were calculated from the kinematic data using theories of steady-state aerodynamics. Total mechanical, power output as a function of forward speed was thus determined. Measurements of the oxygen consumption of free-flying bumblebees and hoverflies were made using a closed circuit wind tunnel. These results were compared with calculated mechanical power requirements to derive the mechanochemical efficiency of the flight system at different flight speeds.