Small-angle X-ray scattering (SAXS) offers a way to examine the overall shape and oligomerization state of biological macromolecules under quasi native conditions in solution. In the past decades, SAXS has become a standard tool for structure biologists due to the availability of high brilliance X-ray sources and the development of data analysis/interpretation methods. Sample handling robots and software pipelines have significantly reduced the time necessary to conduct SAXS experiments. Presently, most synchrotrons feature beamlines dedicated to biological SAXS, and the SAXS-derived models are deposited into dedicated and accessible databases. The size of macromolecules that may be analyzed ranges from small peptides or snippets of nucleic acids to gigadalton large complexes or even entire viruses. Compared to other structural methods, sample preparation is straightforward, and the risk of inducing preparation artefacts is minimal. Very importantly, SAXS is a method of choice to study flexible systems like unfolded or disordered proteins, providing the structural ensembles compatible with the data. Although it may be utilized stand-alone, SAXS profits a lot from available experimental or predicted high-resolution data and information from complementary biophysical methods. Here, we show the basic principles of SAXS and review latest developments in the fields of hybrid modeling and flexible systems.