This article describes the technical challenges that have to be met when implementing the interesting new technology of spectrum pooling. This notion represents the coexistence of two mobile radio systems within the same frequency range. It enables the secondary utilization of already licensed frequency bands as aimed at by several regulatory authorities worldwide. The goal of spectrum pooling is to enhance spectral efficiency by overlaying a new mobile radio system on an existing one without requiring any changes to the actual licensed system. Several demanding tasks originate from this idea. Some of them have been solved in recent research projects. Others are subject to ongoing investigations. Here, the state of the art in spectrum pooling is presented.