The basic principle of describing a structure9s dynamic behaviour in terms of its vibration modes was first explored by Rayleigh over a century ago. Since then, engineers have sought to harness this powerful approach as a means of solving a wide range of practical problems in structural dynamics. About 50 years ago, these efforts were directed towards experimental methods, anticipating the arrival of electronics and computer technologies, and the foundations of experimental modal analysis or modal testing were laid. Now at the turn of the new century, the subject of modal analysis comprises a rich mixture of analytical and experimental techniques, the most powerful of which are those involving both theoretical and practical considerations. In this issue, a series of papers is presented that display the current capabilities and challenges in the subject, with emphasis necessarily on practical issues, and so we start the issue with techniques involving modal testing, interpretation of measured data, and the subsequent application of the test–derived models. It can be concluded that the subject has attained a degree of maturity that renders it a valuable tool for tackling many vibration–related problems but which promises considerable additional advances in the coming years.