This book, like other monographs of the Cellular Organelles series, is not a comprehensive review, but an introduction to the study of cytoskeleton. Accordingly, we describe only the main facts and concepts related to cyto skeleton. Needless to say, selection and interpretation was influenced by the personal interests and opinions of the authors, although we attempted to be as fair as possible. We wished to familiarize the reader not only with well established facts, but with current unsolved problems. Therefore, the words "possibly," "maybe," "not known," and "not clear" are much more frequent in this text than in many others. In accordance with the style of the series, relatively short lists of addi tional readings are given at the end of each chapter; these lists contain mostly the recent reviews and a few original papers describing certain phenomena in detail. Few references are cited in the text; these citations are given to help the reader find the source of certain new data and theories, which are not discussed at length in the reviews. In contrast, many well-established facts and widely known theories are not cited.