A new retrieval of stratus cloud microphysical parameters is developed based on measurements of millimeter-wave radar reflectivity and visible optical depth. The retrieval assumes a lognormal distribution of cloud droplets with no significant amounts of drizzle. The retrieval is formulated in an estimation theory framework, allowing the inclusion of a priori information. This formulation provides quantitative measures of uncertainty in the retrieved quantities and the relative influence of the measurements and a priori data. A radar-only version of the retrieval is described and compared with an analogous radar-only retrieval that does not use an estimation theory approach. The retrievals are shown to agree in the limiting case of infinite a priori uncertainty and negligible measurement and forward model uncertainty. The estimation theory formulation is readily expanded to allow the inclusion of additional data streams. The planned formation flight of CloudSat, ESSP3 (formerly PICASSO-CENA), and Aqua will provide numerous opportunities for the development of combined algorithms using this technique. ¿ 2001 American Geophysical Union