Our knowledge of the physical properties of cometary dust grains derives primarily from measurements of their thermal emission and optical scattering. The infrared data for bright, nonperiodic comets have recently been augmented by observations of fainter periodic comets beyond 1 AU. Small absorbing grains, ≲ 1 μm radius, are necessary to explain the observed infrared flux at wavelengths 3 to 20 μm while small silicate grains are necessary to explain the excess flux near 10 and 18 μm observed in many comets within 1 AU of the sun. These conclusions are consistent with collected micrometeorites and dynamical analysis of comet dust tails. The thermal emission from all well-observed comets at R≦1.5 AU can be fit by a size distribution function with a change in mean particle size less than a factor 2.5.