Millimeter wavelength radars promise certain advantages over lidar techniques for the study of ice clouds, including quantitative data retrieval and the ability to penetrate multiple level clouds. However, the problems in deriving ice cloud content are formidable. In this study the microphysical output of a 2D model with explicit microphysics is converted to radar reflectivity to explore the underlying physics, and problems related to radar ice cloud probing. It is concluded that simple relations based on radar reflectivity will likely yield ambiguous estimates of ice mass content because of characteristic vertical variations in cloud content that evolve with cirrus growth. The weak signals (<~-40 dBZi) produced by the micron-sized particles present in cirrus cloud-top generating layers, and in persisting contrails, make their detection by most radars problematic. ¿ 1998 American Geophysical Union