Aerosol optical depths (τa) are derived operationally for the first time from the MODIS (Moderate Resolution Imaging Spectroradiometer) measurements over vegetated and partially vegetated land at 0.47 and 0.66 ¿m wavelengths. The extensive validation made during July -- September 2000 encompasses 315 co-located τa in space and time derived by MODIS and AERONET (Aerosol Robotic Network) from more than 30 AERONET sites. The lack of AERONET measurements in East Asia, India and Australia makes this validation unavailable for those regions. The MODIS aerosol retrievals, except in coastal zones, are found within the retrieval errors of Δτa = ¿0.05 ¿0.2 τa. The root mean square (RMS) errors are ≤ 0.1 in the continental inland regions and up to 0.3 in the coastal regions (attributed mainly to water contaminated signals). With this validation we believe that MODIS aerosol products can be used quantitatively in many applications with caution for possible residual clouds, snow/ice, and water contamination.