This book is devoted to Large Scale Systems methodologies including decomposition, aggregation, and model reduction techniques. The focus is put on theoretical and practical results resulting from the application of these techniques in the area of stability and decentralized control. Every result is illustrated by examples to facilitate understanding. The appendices provide a collection of ready-to-use packages implementing some algorithms included in the book. Graduate students concerned with system and control theory will be interested in this book, since it offers a global synthesis on the problem of structurally constrained control. The book addresses also scientists and lecturers in the areas of large scale systems and control theory.