**1. Introduction**

Macromolecular complexes have a huge interest in molecular biology. The comprehension of the biological processes in living systems is directly related to the knowledge of the shape and structure of the formed complex and the process of formation. Although X-ray diffraction, Nuclear Magnetic Resonance and cryoEM can provide information on the formed structures, there are several cases where none of those techniques can be applicable. Limitations on molecular weight, the necessity of a well ordered crystal, difficulties on sample preparation etc, are some of the bottle necks of those techniques (Svergun, 2007; Oliveira et al, 2010). Most importantly, in several cases the studies have to be performed directly in solution, with minimum interaction with the studied sample in order to avoid biased results. In this respect, scattering techniques are highly recommended since they allow a study directly in solution in a very non-invasive way. Small angle X-Ray scattering (SAXS) is a standard technique that can be applied to the study of particles in solution, providing information on size, shape, polydispersity, flexibility, oligomerization and aggregation state. Also, it allows real time measurements where the system can be monitored directly in solution, enabling the study of the in situ particle formation (Oliveira et al, 2009). The combination of SAXS and microscopy techniques has been used in several applications due to their complementarity (Oliveira et al, 2010; Andersen et al, 2009). In this chapter some general aspects of Small Angle X-Ray scattering and the state of the art modeling methods will be presented, with several applications.
