The book contains the contributions presented at the NATO ARW on Ferrimagnetic Nano-crystalline and Thin Film Magnetooptical and Microwave Materials (short title: Nano-crystalline and Thin Film Magnetic Oxides) which took place in Sozopol, Bulgaria, Sept. 27 - Oct. 3, 1998. The program of the ARW was consistent with three main areas in the magnetic oxides for microwave and magnetooptical applications : thin films and nano-crystalline ferroxides; magnetic behaviour and applications of oxides with perovskite structures; and nano-sized materials and modeling. The development of planar devices for high-density magnetic and magneto optical recording and microwave integral technologies has led to a substantial growth of the scientific interest in nano-crystallline and thin film magnetic oxides, such as ferrites, manganates and cuprates. The Workshop organizers embarked on the ambitious task to attract the scientists' attention to key problems related to the nano-crystalline state of magnetic oxides and their magnetic, electrical, and optical behaviour. The knowldege of interactions between charge carriers, with phonons, the spin and dipole magnetic moments and the role of the microstructure and magnetic anisotropy is much theoretically studied for magnetic oxides. The workshop program touched not only upon the theoretical aspects, but on the technology and experiments as well. A review of nano-particle technology and future trends was presented. The possibilities of investigating and modeling the domain structure of the magnetic oxide films were demonstrated and discussed. Microwave and magnetooptical applications of ferroxides were also explored, including a discussion on new types of components with nano-size structure.