Generalized measurements are often thought of in the following way. Instead of interacting directly with the measurement apparatus, the system to be measured first interacts with another (microscopic) system, a probe, prepared in a known quantum state. Then the probe is subjected to a projective measurement, and from its outcome one learns something about the original system. If the interaction between the probe and the system of interest is very weak this is called a weak measurement. Sometimes a hybrid measurement is employed, in which, following its interaction with a probe, the original system is subjected to another (typically projective) measurement. Both the weak measurement and the hybrid are examples of generalized measurements, and can be discussed in the manner indicated below. A weak measurement must be carefully distinguished from a weak value; see the comment at the end of this section.