This will be the 3rd volume of a "Light Scattering Reviews" series devoted to current knowledge of light scattering problems and both experimental and theoretical research techniques related to their solution. This volume will cover applications in remote sensing, inverse problems and geophysics, with a particular focus on terrestrial clouds. Clouds have an extremely complex structure and their influence on climate is poorly understood. Therefore, studies of clouds using ground–based, airborne, shipborne and satellite optical instrumentation are of great importance. The theoretical aspects of this problem constitute the main emphasis of this work.