AtLAST is a concept for a next generation, 50-meter class single-dish astronomical observatory. Operating at sub-millimetre and millimetre wavelengths, it will deliver a much needed combination of spatial and spectral resolution, with high mapping speeds and sensitivity to large scale structures that current facilities cannot achieve. With a high throughput, 2 degree field of view and a full complement of advanced instrumentation, including highly multiplexed high-resolution spectrometers, continuum cameras and Integral Field Units, AtLAST will have mapping speeds thousands of times greater than any current or planned facility. It will reach confusion limits below L*, allowing for SDSS-style surveys at long wavelengths and enabling a fundamentally new understanding of the sub-millimetre universe at unprecedented depths.