Underground mining systems rely strongly on the use of pillars to ensure the local and regional stability over the lifetime of the mine. As mining is a dynamic system the pillar stress can change drastically as a result of ongoing mining. Inappropriate pillar design may result in pillar failure and collapse of large mining areas. For this reason this paper summarizes key issues in the rock mechanical design of pillar systems. Therefore the pillar strength and its main influencing parameters as well as the pillar loading mechanisms in partial and total extraction systems are described. Furthermore the consequences of the progress of mining and of pillar failure are investigated by means of numerical simulations and the role of inter-panel pillars and their necessity in case of a pillar failure are outlined.