Research in our group focuses on quantum optical phenomena in complex media. Traditionally, quantum optics studies the interactions between emitters and electromagnetic fields. Early work in this field focused on simple and generic emitters, such as two or few-level systems, embedded in simple photonic media, such as translationally invariant systems or Fabry-Perot like cavities. With the recent progress in micro and nano-fabrication capabilities however, the field saw the introduction of increasingly more complex quantum emitters as well as photonic components. While this provided an unprecedented degree of control over the precise way matter degrees of freedom interact with the omnipresent electromagnetic degrees of freedom, it also lead to formidable theoretical challenges in the description of the systems at hand. Our research aims at a realistic description of the physics of state-of-the art photonic media as well as novel emitter systems. Research projects in our group have a well-balanced applied and fundamental physics component. On the one hand, our research contributes directly to the development of more efficient opto-electronic devices such as power-efficient, highly functional lasers in micro- and nano-scale, single-photon emitters and, in the long run, quantum simulators. On the other hand, we address fundamental theoretical issues involved in developing the necessary numerical and analytical tools in the realistic description of these systems which are situated at the interface of quantum optics, non-linear optics and condensed matter physics.