This book studies the crash response of thin-walled beams with different cross sections. Two collapse behaviors are studied in this book, the axial collapse and the bending collapse. Mathematical equations are derived to predict and describe the thin-walled beams' collapse mechanisms. Based on the present collapse theories, simplified finite element (FE) models and modeling methodologies are developed for different types of thin-walled beams. The developed models are validated through crashworthiness analysis and the results agree very well to those obtained from the detailed FE models. As demonstrated in this book, compared to the detailed models, the developed simplified models have reduced size and consist of less number of elements thereby can save much more modeling labors and computing time. Such advantages make the simplified models most suitable for computer simulation and product design. For an example, the simplified models and modeling methods presented in this book can be applied in optimum design of the thin-walled beams.