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. It is one of the central aims of this Heraeus seminar to bring both groups of researchers closer together by providing a stage for intra- and interdisciplinary scientific exchange. Besides, in view of the upcoming European Spallation Source, a further focus will be on future challenges related to neutron instrumentation, sample environment, and neutron data analysis.