Pegg is internationally renowned for his breadth of theoretical work in fundamental physics, which ranges from the interaction of electromagnetic radiation with atoms and nuclei to the nature of time. In particular, he originated a powerful unitary transformation technique for investigating field-atom interactions. With experimental colleagues he invented a variety of pulse sequences in NMR which are now routinely used worldwide. In quantum optics he formulated, with Barnett, the quantum theory of phase based on a Hermitian operator, a procedure which had been deemed not possible but is now well accepted for examining the phase properties of light.