Qualitative researchers often use other principles for judging the quality of their study than quantitative researchers. This inhibits a straightforward assessment of the quality and comparability of different types of studies, as well as decision-making about their usefulness for further research and practice. In this article, we question the use of alternative criteria, and argue for one coherent and inclusive framework of quality criteria for both qualitative and quantitative studies. We developed such a framework, based on a comparison and operationalization of different criteria, and the elaboration of procedures to realize them. Its usability is demonstrated by the application in a qualitative case study research and a mixed-methods study. This framework enhances advancing and judging the basic quality of any type of scientific research, promoting the assessment of the quality and hence usability of studies for further research or decision-making about practice.