Background and current status    The international interdisciplinary community of scientists is joined within the annual workshops MUSCLE (MUltiple SCattering Lidar Experiments) for about twenty years. This community combines scientists around the idea to apply laser radar (lidar) radiation for remote sensing of the atmosphere, especially in the case of large optical depths (clouds and fogs) and/or nonspherical scattering particles (ice crystal clouds). The ground-based, airborne, as well as spaceborne lidars are of interest.
   These problems are of general physical interest, too, since the same problems appear in other areas of physics dealing with propagation and scattering of any radiation in discrete media. Complexity of the problems calls for a considerable part of mathematicians simulating the multiple scattering of radiation, as a rule, by the Monte Carlo procedures.

The expected contribution to advance of the specific scientific area    In the beginning of the MUSCLE meetings, the central idea was to investigate the multiple scattered lidar returns by use of detectors with variable fields of view. A number of instruments based on this idea were fabricated and explored (Germany, Canada). A possibility to use the lidars instead of radars for consumer car navigation systems was studied. A number of algorithms and retrieval procedures were worked out. In particular, the software toolboxes Backscatter Lidar Virtual Instrument and Virtual Lab for Light Scattering and Radiative Transfer have been developed (Germany).
   Recently, new tendencies inside the MUSCLE community appear. First, launch of the spaceborne lidar LITE in 1994 emphasized an essential role of multiple scattering in spaceborne lidar returns. The launch of the next spaceborne lidar CALIPSO (USA - France) planned in 2005 put in the forefront the problem of numerical assessment and interpretation of the CALIPSO signals.
   Second, new fast detectors of light offered a possibility to measure not only temporal but spatial-temporal structure of backscattered lidar radiation. This is a new and promising topic in the MUSCLE meetings.
   Third, it is recognized that the state-of-the-art numerical models of climate changes and of weather forecasting need scattering coefficients for cirrus clouds consisting of ice crystal particles. The problem of both single and multiple scattering by nonspherical ice crystal particles is promising also.