This WE-Heraeus-Seminar focuses on the technique of magnetic small angle neutron scattering, which is one of the most important methods for magnetic microstructure determination in condensed-matter physics and materials science. Magnetic SANS provides access to bulk properties and yields, quite uniquely, information on the mesoscopic length scale (roughly 1-1000 nm). This is an important size regime where many macroscopic materials properties are realized. Currently, the magnetic SANS community can be roughly subdivided into two larger groups: the research of one group is anchored in the domain of condensed-matter physics with a focus on fundamental questions such as skyrmion crystals and topological spin structures, complex long-range-ordered spin structures, and vortex lattices in superconductors, while the second group of scientists employs the magnetic SANS method for scrutinizing nanoscale magnetism in terms of a micromagnetic continuum description; materials classes which are studied are e.g. permanent magnets, magnetic steels, nanoparticles and ferrofluids, or complex alloys.