The solid-liquid suspensions are frequently used in industrial processes. These suspensions usually contain aggregates made up of solid primary particles. Many characterization tools of these suspensions are based on light scattering (Mie theory). However, the Mie theory (1908) is not always applicable to the practical problems since the scatterer must be a homogeneous sphere. The ordinary particle sizers that use this theory do not make it possible to measure the non-spherical particle geometrical characteristics. Extensions of the Mie theory for arbitrary shaped particles or particle aggregates are available nowadays (T-Matrix method, Generalized Multiparticle Mie (GMM)-solution, etc.). But the computing times of the optical properties via these exact theories do not allow for a real-time analysis. This chapter is thus dedicated towards the search of approximate methods for the estimate of aggregate optical properties, particularly their scattering cross section.