A review is made of developments in wavelength measurement practice over the last 5 years. The rapid development of single-frequency tunable dye lasers has led to a requirement for quick and easy wavelength measurement at accuracies of 1 part in 106 or better. This has stimulated the development of moving-carriage interferometers and of methods of image sensing with detector arrays or vidicon tubes. There has also been progress in the attainment of very high accuracy in wavelength comparisons of stabilized lasers, now nearly matching the laser reproducibilities of a few parts in 1011. The limitations to accuracy seem to be predominantly the defects of optical surfaces and uncertainties arising from diffraction effects.