This book consists of four chapters to address at different modeling levels for different nanoscale MOS structures (Single- and Multi-Gate MOSFETs). The collection of these chapters in the book are attempted to provide a comprehensive coverage on the different levels of electrostatics and transport modeling for these devices, and relationships between them. In particular, the issue of quantum transport approaches, analytical predictive 2D/3D modeling and design-oriented compact modeling. It should be of interests to researchers working on modeling at any level, to provide them with a clear explanation of theapproaches used and the links with modeling techniques for either higher or lower levels.