At the highest level of description, this book is about dimensionality reduction for data mining. It focuses on dimensional reduction on Small, Medium and very large Scale data. This book is not intended as an introduction to Feature selection, although we do provide the background necessary in several areas in order to facilitate the reader's comprehension of their respective roles in data mining. This book reviews state-of-the-art dimensionality reduction techniques for analyzing enormous quantities of raw data in high-dimensional data spaces to extract new information useful to the decision-making process.