This book concentrates on both understanding and development of nanocrystalline materials. The original relation which connects grain size and strength known as the Hall-Petch relation is studied in the nanometer grain size region. The breakdown of such a relation is a challenge. Why and how to overcome? Is the dislocation mechanism still operating when the grain size is very small approaching the amorphous limit? How do we go from the microstructure information to the continuum description of the mechanical properties.