Measurement uncertainty is a statistical parameter which describes the possible fluctuations of the result of a measurement. It is not a mere repeatability but it is at least as high as the intra-laboratory reproducibility. If it is an attribute of a general analytical test procedure it is at least as high as the inter-laboratory reproducibility. Measurement uncertainty can be determined by the addition of the variances of the individual steps of the test procedure or by an approach which starts with one of the above-mentioned reproducibilities. Any measurement uncertainty should be kept low but it is objectionable to state too low a value, e.g. by falsely reporting mere repeatability data instead of properly determined uncertainty data. Some good working principles can help to obtain low measurement uncertainties.