During the Midlatitude Cirrus Experiment (MidCiX), in situ measurements of cirrus cloud microphysical properties were made from aboard the NASA WB-57F aircraft in conjunction with Terra and Aqua satellite overpasses. These in situ data are directly compared to retrievals of cirrus visible optical thickness (tau), effective size (D-e), and ice water path (IWP) from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard Terra and Aqua. The MODIS data considered here include both the operational retrieval (MOD06) and visible optical thickness retrieved from the 1.38-mu m channel. Instruments aboard the WB-57F included three bulk probes for measuring ice water content (IWC), a cloud integrating nephelometer for measuring the visible extinction coefficient (beta), and four optical particle probe instruments from which beta and D-e are inferred. In situ beta( IWC) data taken during vertical spiral profiles through cirrus are integrated to get tau(IWP) for comparison with MODIS values, and a methodology for comparing satellite and aircraft data is developed and illustrated using several case studies from MidCiX. It is found that the presence of cirrus overlapping low cloud layers significantly biases the MODIS operational tau to high values. For single-layer cirrus cases, the in situ IWP agree with MODIS values to within 20%, on average. The in situ tau/D-e differ from MODIS values in a manner that is roughly consistent with previous claims of particle shattering on aircraft inlets, although the magnitude of the differences is less than expected, and biases in the MODIS retrievals cannot be ruled out.