The lack of evidence-based quantitative studies prevents further progress in mentoring research. In particular, standardized diagnostic instruments facilitating the exploration, evaluation and production of structured feedback for mentors and mentees are urgently needed. The purpose of this study was to investigate the factors and levels that are crucial to the process of mentoring. The study has two objectives: First, to expand the present empirical knowledge of basic dimensions and mentoring styles by developing the first German-language inventory, and second, to examine how the dimensions of the inventory are related to other qualities in the mentoring process. The data were collected at three universities in Austria during and after the school practice periods (student teaching) of advanced student teachers who were under the guidance of mentor teachers. Over the course of the study, 405 mentees (future teachers) evaluated 205 mentors. In order to gather information on mentoring dimensions, a specially designed German-language questionnaire with 53 items was utilized to assess how often certain mentoring behaviors were experienced. Five factors, some of which were validated by independent variables, were identified through an exploratory factor analysis: “Professional Support”, “Collegiality”, “Working Levels”, “Directiveness” and “Confidence”. The resulting inventory promoted two objectives: a theory-focused goal to encourage further research on the complexity of mentoring processes; and a practical goal, the creation of a tool for collaborative reflection between mentor and mentee. The results indicated that mentoring must be conceptualized as a professional practice that should entail specific resources and guidelin