Small-angle X-ray scattering (SAXS) allows one to study native biological macromolecules, from individual proteins to large complexes, in solution under nearly physiological conditions. SAXS not only provides low resolution three-dimensional models of particle shapes but yields answers to important functional questions. Thus, kinetic SAXS experiments allow one to analyze structural changes in response to variations in external conditions, protein-protein and protein-ligand interactions, and to study kinetics of assembly/dissociation or folding/unfolding. Fundamental biological processes such as cell-cycle control, signalling, DNA duplication, gene expression and regulation, some metabolic pathways, depend on supra-molecular assemblies, and their changes over time. There are objective difficulties to study such complex systems, especially their dynamic changes by other structural techniques like spectroscopy, NMR and X-ray crystallography. Furthermore, for macromolecules with molecular masses of a few hundred kDa - too large for NMR and too small for cryo- electron microscopy (EM) - SAXS remains effectively the only method to obtain their shape in native conditions.