This IMA Volume in Mathematics and its Applications TWIST MAPPINGS AND THEIR APPLICATIONS is based on the proceedings of a workshop which was an integral part of the 1989- 90 IMA program on "Dynamical Systems and their Applications." The workshop brought together many of the leading figures in the modern study of twist maps. We thank Shui-Nee Chow, Martin Golubitsky, Richard McGehee, Ken Meyer, Jiirgen Moser, Clark Robinson, George R. Sell, and Eduard Zehnder for organizing the meeting and, especially, Richard McGehee and Ken Meyer for editing the volume. A vner Friedman Willard Miller, Jr....