Physical principles and design criteria for a large-aperture scintillometer conceived at the NOAA Wave Propagation Laboratory in Boulder, Colorado are presented. This instrument measures intensity fluctuations of a collimated beam of incoherent light at spatial wavelengths within the inertial subrange. Together with measurements of mean windspeed and rule-of-thumb estimates of the surface roughness, these optical scintillation measurements yield sensible heat flux and friction velocity (u*) via a simple iterative procedure. A maximum path-length of some 1.7 km is calculated for near-surface measurements (1-2 m height) of the path-averaged sensible heat flux (H) and this range increases with increasing height as z 1 2 A field test over pasture with a transmitter-receiver separation of 350 m gave estimates of H and u* within 7 and 1% of those determined directly by eddy-correlation. Over the measured ranges (0 < H < 350 W m-2 and 0.05 < u* < 0.6 m s-1), residual standard deviations were 25 W m-2 and 0.04 m s-1. Given these results, the ability of the large-aperture scintillometer to provide estimates of fluxes over homogeneous surfaces seems assured.