Despite the importance of scientific inquiry in science education, researchers and educators disagree considerably regarding what features define this instructional approach. While a large body of literature addresses theoretical considerations, numerous empirical studies investigate scientific inquiry on quite different levels of detail and also on different theoretical grounds. Here, only little systematic research has analysed the different conceptualisations and usages of the overarching construct of scientific inquiry in detail. To close this gap, a review of the research literature on scientific inquiry was conducted based on a widespread approach to defining scientific inquiry as activities that students engage in. The main goal is to provide a systematic overview about the range and spectrum of definitions and operationalisations used with regard to single activities of the inquiry process in empirical studies. The findings from the review first and foremost illustrate the variability in the ways these activities have been operationalised and implemented. For each activity, studies differ significantly not only with respect to the focus, explicitness and comprehensiveness of their operationalisations but also with regard to the consistency of their implementation in the form of instructional or interventional components in the study and/or in the focus of the assessment of student performance. This has significant implications regarding the validity and comparability of results obtained in different studies, e.g. in the context of discussions concerning the effectiveness of inquiry-based instruction. In addition, the interrelation between scientific inquiry, scientific knowledge and the nature of science seems to be underexplored. The conclusions make the case for further theoretical work as well as empirical research.