Self-interference, i.e. the interference of a quantum object with itself, is a fascinating phenomenon of quantum mechanics, which we discuss below in terms of the interaction-free quantum measurement. The experiment is based on the principle of the Mach–Zehnder interferometer (MZI). It shows the existence of quantum superpositions as clearly as the famous double-slit experiment, but it is by comparison formally and experimentally much ‘handier’, so that it is increasingly finding its way into textbooks. At the same time, it also allows for the treatment of further-reaching questions. That is why we meet the MZI not only in many modern basic experiments, but also for example in the field of quantum information, where we can realize basic functions of the quantum computer by means of the MZI and its components (see the closing remarks to this chapter).