During the last three decades, interest in the field of interaction of microwaves with ferrimagnetics has steadily increased. Investigations in tlris field have led to the development of a number of devices used for a variety of applications. The initial emphasis of the investigators was on the microwave behavior of ferrimagnetics placed in cavities and metallic waveguides and associated devices. This work has been presented in various books, monographs, and reviews written during the sixties. In recent years, interest in microwave propagation in ferrimagnetics has shifted from loaded waveguides to relatively new areas, e. g. , magnetostatic and magnetoelastic waves in layered structures, microwave propagation in ferrimagnetic strip lines and microstrips, etc. Such investigations are important from the viewpoint of devices such as delay lines, filters, convolvers, guided wave amplifiers, striplines, and microstrip phase shif ters, circulators, edge guided mode isolators, etc. As such, we feit the need for a text (meant for graduate students starting work in these areas as weil as practicing electrical engineers and applied physicists) which presents a coherent account of the various aspects of propagation of microwaves (electromagnetic as weil as magnetoelastic) in biased ferrimagnetics and discusses the relatively recent developments in the theory and operation of the aforementioned devices, and this book is the result. A biased ferrimagnetic is, in the mathematical sense, a complicated medium, electromagnetically as weil as elastically.