Starting out from Hattie's focus on making student learning visible, we argue that this is also crucial to teaching processes. For this purpose, we have developed a system of ques­tionnaires, software and other materials which helps teachers to gather an empirical data base in order to monitor and improve their instruction. Based on modern research on teaching effectiveness, and on a concrete lesson, the core of the instrument consists of tools for a systematic comparison of different perspectives: the teacher's self-report, rating of an observing colleague, and students' feedback.