The Atmospheric Radiation Measurement (ARM) Program installed the first operational millimeter cloud radar (MMCR) at its Southern Great Plains in 1996. Since then, ARM has installed four other MMCR’s in Barrow Alaska (1998), Nauru (1998), Manus Island, Papua New Guinea (1999) and Darwin, Australia (2002). These five radars comprise the only continuously operating mm-wave radars in the world and are the core instruments for ground-based remote sensing of cloud properties at the ARM sites. In addition to collecting the data, the ARM program has sponsored a broad range of research on the use of mm radar data to retrieve cloud properties. Some of the products of this research are the first-ever climatologies of the vertical distribution of clouds, algorithms to retrieve microphysical properties in stratiform clouds, and correlations of cloud properties with atmospheric thermodynamical and dynamical properties. In addition, these measured properties have been used to validate satellite retrievals, cloud resolving model output, and weather forecast model output. Climatically, clouds are the most important regulator of the radiation budget of the atmosphere. Thus, the MMCR-derived cloud properties taken in conjunction with surface and top-of-atmosphere radiation measurements can be used to understand cloud radiative forcing and atmospheric heating profiles. This talk will provide a summary overview of the MMCR operations, derived properties, and applications to atmospheric radiation budget and heating rates.