Light is a primary tool to investigate and control matter. Due to the developments in the last years, x-ray radiation sources like 3rd generation synchrotrons and free electron lasers provide light with unprecedented quality, which allows to study structures in largely unexplored energy regimes. However, it is still challenging to exploit the beneficial concepts based on coherence and interference, which are well established at lower frequency ranges. Therefore, a platform in which those ideas can be realized with x-rays would be desirable for our fundamental understanding as well as for related applications. A promising platform in which quantum optical phenomena can be realized are planar x-ray cavities with embedded Mössbauer nuclei (Fig. 1). Effects such as cooperative Lamb shifts [1] or vacuuminduced coherences [2] have been recently observed therein. These systems can be well described with the concepts of cavity-QED [3], which further demonstrates the quantum character of the occurring interactions. In this talk I will review our recent theoretical and experimental progress in the field of x-ray quantum optics in such cavities. The basic ingredients for the theoretical modeling are discussed and applications such as line shape control and interferometer setups are presented together with recent experimental results.