Preface: Quo vadis Quantum Optics?

A editorial overview of Optics Communications' special issue, called 'Quo vadis Quantum Optics?' is presented. The special issue was published to recognize the efforts of Krzysztof in the field. The issue describes the high quality of research Krzysztof has initiated in the field of quantum optics. Krzysztof has also made significant contributions in the field of thermodynamics of quantum optical systems. This subject is one of the first instances of quantum many-body physics in quantum optical systems. One of the most important areas of Krzysztof's deals with measurement and detection theory, specifically the problem of quantum measurements. He has made seminal contributions to this field starting with the famous paper with Joe Eberly on the time-dependent spectrum to experimental papers on the direct probing of quantum phase space by photon counting with Konrad Banaszek.