For an efficient and safe underground mining operation, the use of backfill is often required. “Backfill” describes any type of filling material which is introduced into underground openings for a variety of reasons, the stabilization of the underground openings being usually the main reason for filling activities. In general, it is known that backfill has a positive effect on the stability of the underground openings as well as on pillar stability, but questions about the occurring phenomena as well as on backfill effect in mining activities of alpine deposits are largely unexplored. Thus, the objective of the present work is to investigate the interaction of rock mass and backfill, especially concerning mining activities in the alpine region. In this context, the focus will be placed on the influence of backfill on the stability of the rock mass, in particular of slender pillars commonly used in alpine mining. In this context, also the backfill properties, in particular the binding agent addition to the backfill material is of great importance, since a variety of different backfill types are used, but there is no information on the exact effect of different backfill properties. To answer the quoted questions, the basics of backfill technology were discussed in a first step and a literature review of existing research was carried out in order to raise open questions and to integrate them into the research activities. Underground measurements and laboratory tests were carried out, which, in combination, should contribute to the current state of knowledge in the field of the interaction of rock mass and backfill. The in situ investigations were carried out in the underground mine Breitenau, a magnesite mine located in the Eastern Alps, where magnesite is extracted by means of open stoping with cemented backfill in the lowest mining level. The laboratory tests are model pillar tests in which model pillars out of cement mortar are tested in a steel frame, which constitutes the rock mass. The model pillars were surrounded by three different backfill materials and loaded under uniaxial stress. On the basis of the underground measurements and the laboratory experiments, the positive backfill effect on the stability of slender pillars could be clearly demonstrated. The influence of backfill on the maximum strength of the pillars is likely to be of minor importance but shows a pronounced stabilization in the post-failure behavior. All underground measurements showed that there is immediate stabilization of the rock mass after backfill placement. Concerning backfill properties, the uniaxial compressive strength plays a minor role for the stabilization of the rock mass, since its stabilizing effect could be perceived even before curing of the backfill body. Here, a highly compacted and cohesive backfill is of advantage, as it prevents slipping along fracture surfaces and thus increases the residual strength of the pillars. On the basis of the underground measurements and the laboratory investigations a simple model for the backfill effect was created, which also shows that already small backfill pressures are sufficient to prevent the slipping of rock wedges out of the rock mass.