Through his outstanding experimental and analytical skills, David Hanna has made a considerable contribution to the understanding of laser physics and the control of laser devices. He has applied a thorough, quantitative treatment to many important topics (for example, relaxation oscillations, resonators, mode selection, nonlinear optics, parametric oscillators and waveguide lasers) which were previously inadequately understood. His research on fibre lasers, in particular the Ytterbium-doped lasers, has contributed greatly to a new generation of lasers which are compact, stable, highly efficient and capable of operating at very high powers.