“Measurement is an art” is a well-known saying about experimental physics. Even if this saying should not be taken literally, it points to an important problem. Anyone who carries out a measurement may hope to obtain the true value of a quantity as a result, but unfortunately this is not the case. There are many reasons why the measured value will deviate from the true value: environmental influences disturb the measurement, the measuring devices only have a limited accuracy, to name just two examples. The experimenter must live with the fact that the measurement result does not reflect the true value. The art lies in getting as close as possible to the true value. In this chapter, we give a very brief introduction to the treatment of uncertainties (errors) of measurements.