Professor Mould is among the world's leading astronomers. He is distinguished for his studies of stellar populations and observational cosmology. He was the first to show that the dwarf spheroidal galaxies had a star formation history extending over billions of years, in contrast to the view at the time that these galaxies are pure sample of the oldest stars in the Universe. He pioneered the dynamical study of velocity deviations from the smooth Hubble expansion in the local Universe. He leads a very large international project to measure the Hubble Constant with the Hubble Space Telescope.