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Temperature measurements below 1 K are relatively easy if they are only made to provide a scaling parameter for some other physical quantity because there is a multitude of practical methods to use. That there is problem of relating these measurements to the thermodynamic basis of temperature, however, becomes obvious when any two methods are applied simultaneously and produce discrepant results. Solving this problem requires identification and correction of systematic errors by analysing their sources for each thermometer in detail. In the very few cases where this procedure is successful, one obtains a primary thermometer. Since no primary thermometer is available that is also easily reproducible, one has to leave the firm thermodynamic basis frequently in favour of the reliability of secondary thermometers.This paper reviews the current status of thermometry from a metrological point of view. It emphasizes the relation to thermodynamic temperature without completely ignoring practical questions. Thus principles and limitations are discussed in detail for primary thermometers as well as for those secondary thermometers that can be employed to extend the temperature range by extrapolation. Practical thermometry is covered in a few instructive examples which illustrate the effects of experimental problems on temperature measurement. A considerable fraction of the paper is devoted to temperature reference standards, and the actual state of their underlying scales, that are used to reproduce accurately temperature values for calibration of secondary thermometers without running primary thermometers.

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