Interfacial structures on the nanoscale are crucial for a wide field of applications ranging from biological membranes in life sciences through organic solar cells and industrial coatings in soft matter to hard matter structures. The latter include thin film coatings and materials such as magnetic nanoparticles and topological structures like magnetic skyrmions. Advancing such application rests on the ability to reveal structures at hidden interfaces at different length scales. Techniques like scanning probe microscopy provide information about surface structures and topology with high precision, but are limited to the probe interacting with the surface. The high penetration power of neutrons, their sensitivity to light elements and the capability to probe magnetic structures makes grazing incidence small-angle neutron scattering (GISANS) a valuable, but yet underexploited, tool for organic materials and condensed matter systems and, in particular, to study buried interfaces. Together with specular and off specular neutron reflectometry, interfacial structures on length scales from a few nanometres up to micrometres can be revealed. Apart from giving a comprehensive introduction to the GISANS technique, we will highlight the importance of interfacial layer structures in different fields and how recent developments in the surface techniques have revealed common features in a range of systems. This includes the formation, structure, and topology of the interfaces of and between materials, which are controlled by the same fundamental molecular and colloidal forces.