The seminar for which the proceedings are published here evolved from a cooperative research program on bilinear systems and applications to immunology at the Oregon State University and at the University of Rome. The topics include more general forms of variable structure systems which may be divided into categories of mathematical system theory, economic applications and biological applications. Throughout the seminar there was emphasis on the integration of theory and app- cation. In most cases, theoretical derivations are motivated by their need to solve practical problems. In reading the proceedings, it becomes apparent that bilinear systems, quadratic systems and more general variable structure or adaptive systems become natural models in many cases and excellent approximations in others. It is seen that linear systems have very limited use particularly in economics and biology. Variable structure systems are analyzed in terms of structure, volterra kernels, system modelling, parameter identification, - controllability and Lie algebra to mention a few. Certainly, it ia nbt possible to present a complete tre- ment of these numerous topics, but at the Same time the unifying power of the systems approach and variable structure systems is show

The proceedings of the Second US-Italy Seminar on Variable Structure Systems is published in this volume. Like the first seminar, its conception evolved from common research interests on bilinear systems at the Istituto di Automatica of Rome University and at the Electrical and Computer Engineering Department of Oregon State University. Again, the seminar was focused on variable structure systems in general. In this case, however, emphasis is given to applications in biology and economics along with theoretical investi gations which are so necessary to establish a unified theory and to motivate further developments in these applications of social significance. By bringing together the talents of social and biological scientists with those of engineers and mathematicians from throughout Italy and the United States, the seminar was intended to yield a cross-pollination of significant results and a base for more meaningful future research. The editors are encouraged by the progress, with which they hope the reader will agree, is made in this direction. No pretense is made, however, that completely satisfactory integration of theore tical results and applications has been accomplished at this time. Among the more important conclusions which have resulted from this seminar are that bilinear and more general variable structure models arise in a natural manner from basic principles for certain biological and economic processes."