To make measurements comparable, the quality must be stated by specifying the measurement uncertainty. The ISO/BIPM "Guide to the Expression of Uncertainty in Measurement" published in 1993, normally abbreviated to GUM, introduced a unified method of determining and specifying the measurement uncertainty. This method was adopted by the calibration laboratories worldwide. Its application, however, requires some mathematical knowledge. The simplified approach used in the following chapters makes the measurement uncertainty clearly understandable for everyone who uses temperature measurement systems.Mistakes made when installing temperature sensors and connecting the evaluation electronics lead to increased measurement error.In addition, there are the measurement uncertainty components of the sensor and evaluation electronics themselves. After an explanation of the measurement uncertainty components, a number of worked examples are set out.A knowledge of the measurement uncertainty components and their order of magnitude enables the user to reduce individual components by changing the installation conditions or selecting a different instrument. The crucial factor is always the measurement uncertainty required for a measurement task. Where, for example, a standard specifies limits for the deviation of the temperature from the nominal value, the measurement uncertainty of the measurement method used should not exceed 1/3 of the limit.