NASA’s MODerate-resolution Imaging Spectroradiometers (MODIS) have been observing the Earth from polar orbit, from Terra since early 2000 and from Aqua since mid 2002. We have applied a consistent retrieval and processing algorithm to the entire time series of both MODISs, deriving the Collection 5 (C005) dark-target aerosol products. Here, we co-locate the MODIS field of view aerosol retrievals (Level 2) with AERONET sunphotometer measurements at nearly 300 sites, resulting in over 100,000 matched pairs. Over land and ocean separately, we characterize the expected uncertainty (EU) for particular MODIS -derived aerosol parameters. We demonstrate that global EU for total and spectral aerosol optical depth (AOD or τ) is ±(0.05+0.15τ) and ±(0.04+0.05τ) over dark land and ocean respectively. We identify systematic errors at particular sites and seasons. In some cases AOD agreement is good at all wavelengths, in others the AOD is accurate at one wavelength, but spectral dependencies are not well captured. For yet others, AOD in all wavelengths compares poorly. We also assess MODIS-derived aerosol size parameters. With EU of ±0.45, MODIS has little skill at deriving Ångström exponent (α) over land, globally, but has qualitative accuracy over specific sites. Over the global ocean, α is much better constrained (EU = ±0.3), although there are clear biases for regions known for dust or higher aerosol absorption. A better alternative for estimating submicron-sized aerosol contribution is the fine aerosol AOD (fAOD), which is shown to have EU over land and ocean more similar to the total AOD. Finally, we define the Fraction of EU (FEU) for AOD, to characterize how the sensors may be changing over time. We find that the MODIS/AERONET comparison is consistent throughout the entire mission for both Terra and Aqua over ocean, as well as for Aqua over land. There is a systematic change for Terra over land that we believe to be a result of calibration uncertainty.