Through his work in seismology, Guy Masters has made major contributions to the use of long period waves in determining the Earth’s structure. He made the definitive estimate of the density difference between solid inner and liquid outer cores, thereby proving a difference in chemical composition between the two, and worked out important consequences for the Earth’s thermal evolution. He was first to establish the existence of large-scale, non-spherical structure in the Earth’s mantle. He was first to observe anomalous splitting of free oscillations, later attributed to anisotropy of the inner core, and coupling between free oscillations caused by the Earth’s rotation. Much later, he established a tight limit on the rotation speed of the inner core. He has pioneered many new techniques for analysing seismic data and used them to produce Earth models for all the seismic parameters.