A series of cirrus cloud simulations performed using a model with explicit cloud microphysics is applied to testing ice water content retrieval algorithms based on millimeter-wave radar reflectivity measurements. The simulated ice particle size spectra over a 12-h growth/dissipation life cycle are converted to equivalent radar reflectivity factors Ze and visible optical extinction coefficients , which are used as a test dataset to intercompare the results of various algorithms. This approach shows that radar Ze-only approaches suffer from significant problems related to basic temperature-dependent cirrus cloud processes, although most algorithms work well under limited conditions (presumably similar to those of the empirical datasets from which each was derived). However, when lidar or radiometric measurements of or cloud optical depth are used to constrain the radar data, excellent agreement with the modeled contents can be achieved under the conditions simulated. Implications for the satellite-based active remote sensing of cirrus clouds are discussed. In addition to showing the utility of sophisticated cloud-resolving models for testing remote sensing algorithms, the results of the simulations for cloud-top temperatures of 50°, 60°, and 70°C illustrate some fundamental properties of cirrus clouds that are regulated by the adiabatic process.