Small-angle scattering of X-rays (SAXS) is an established method for the low-resolution structural characterization of biological macromolecules in solution. The technique provides three-dimensional low-resolution structures, using ab initio and rigid body modeling, and allow one to assess the oligomeric state of proteins and protein complexes. In addition, SAXS is a powerful tool for structure validation and the quantitative analysis of flexible systems, and is highly complementary to the high resolution methods of X-ray crystallography and NMR. At present, SAXS analysis methods have reached an advanced state, allowing for automated and rapid characterization of protein solutions in terms of low-resolution models, quaternary structure and oligomeric composition. In this communication, main approaches to the characterization of proteins and protein complexes using SAXS are reviewed. The tools for the analysis of proteins in solution are presented, and the impact that these tools have made in modern structural biology is discussed.