WALES (Water Vapour and Lidar Experiment in Space) was an ESA Earth Explorer candidate concept for global, high-accuracy and high-vertical-resolution water-vapour profiles through the troposphere and lower stratosphere. Its active four-wavelength differential-absorption lidar would overcome the uneven coverage and bias of radiosondes and passive sensors, support climate and numerical-weather-prediction models, and provide cloud, aerosol, boundary-layer and surface-reflectance spin-offs. ESA assessment material documents WALES as a research concept rather than an implemented programme.