Small-angle scattering of X-rays (SAXS) and neutrons (SANS) is an established method for the structural characterization of biological objects in a broad size range from individual macromolecules (proteins, nucleic acids, lipids) to large macromolecular complexes. SAXS/SANS is complementary to the high resolution methods of X-ray crystallography and nuclear magnetic resonance, allowing for hybrid modeling and also accounting for available biophysical and biochemicaldata. Quantitative characterization of flexible macromolecular systems and mixtures has recently become possible. SAXS/SANS measurements can be easily performed in different conditions by adding ligands or binding partners, and by changing physical and/or chemical characteristics of the solvent toprovide information on the structural responses. The technique provides kinetic information about processes like folding and assembly and also allows one to analyze macromolecular interactions. The major factors promoting the increasingly active use of SAXS/SANS are modern high brilliance X-ray and neutron sources, novel data analysis methods, and automation of the experiment, data processing and interpretation. In this book, following the presentation of the basics of scattering from isotropic macromolecular solutions, modern instrumentation, experimental practice and advanced analysis techniques are explained. Advantages of X-rays (rapid data collection, small sample volumes) and of neutrons (contrast variation by hydrogen/deuterium exchange) are specifically highlighted. Examples of applications of the technique to different macromolecular systems are considered with specific emphasis on thesynergistic use of SAXS/SANS with other structural, biophysical and computational techniques.