Erta 'Ale volcano, sited within the Afar Triangle of Ethiopia, is one of the least frequented, perennially active, subaerial volcanoes. By compiling a time series of Landsat MSS and TM, JERS-1, SPOT, and AVHRR digital imagery, and space-borne photographic data, we have been able to constrain the activity of this volcano, important for its geodynamic setting, during the long period since 1974 when volcanological investigations effectively ceased. Existing techniques for infrared thermometry have been modified to cope with saturation of the short wavelength infrared Landsat TM band 5 and 7 sensors, enabling derivation of thermal fluxes from Erta 'Ale's active lava lakes. Lake levels have been estimated from measurements of shadow lengths cast by the crater rim. Changes in caldera and flank reflectances identify new lava flows whose areas and volumes we have constrained in order to derive eruption magnitudes and effusion rates. Between 1968 and 1974, approximately 3 1010 kg of lava was erupted at peak discharge rates exceeding 400kg s -1, though much of this subsequently drained back or subsided rigidly. No post-1974 overflows were detected in the imagery, although thermal (100-400 MW) output from the lava lakes, and gas emissions appear to have been sustained up to the time of writing. The longevity of the lava lakes provides evidence for convective circulation between the lakes and a deeper magma reservoir. The high heat flux and low exogenous growth rates ( 10kg s -1 integrated over the last thirty years) are indicative of a volcano that 'grows' largely by magmatic intrusion, which is consistent with the formation of new igneous crust in the extensional tectonic environment of northern Afar.