  The principles of scale setting in lattice QCD as well as the advantages and
disadvantages of various commonly used scales are discussed. After listing
criteria for good scales, I concentrate on the main presently used ones with an
emphasis on scales derived from the Yang-Mills gradient flow. For these I
discuss discretisation errors, statistical precision and mass effects. A short
review on numerical results also brings me to an unpleasant disagreement which
remains to be explained.
