Aim in writing this book has been to make these developments accessible to a wider circle of people, and to encourage more research workers to take up this difficult but interesting and rewarding subject. Author have had in mind readers both with engineering and with mathematical backgrounds. For the sake of the engineers, author have kept the mathematics as simple as is consistent with a proper exposition of the subject and mathematical ideas which such readers may not have met before are explained in the first two Chapters. For mathematical readers, author have tried to explain which aspects of the subject are of particular concern to engineers, and to give a little of the engineering background.