This book, written by an international team of experts, introduces the reader to various aspects of complexity theory and its applications. It illustrates the latest trends in science to go beyond the mechanistic Newtonian view of the world by shifting the focus to self-organization, adaptation, and emergent phenomena. The authors discuss these properties of complex systems in biology, ecology and chemistry along with the structure and interconnectedness of the "layers" of complexity. The qualitative description is complemented by a discussion of methods for complexity quantification. Networks are covered in detail as a universal language of the complex world. General readers, as well as undergraduate and graduate students and researchers in life sciences, chemistry, and nanotechnology will find this book to be of great interest.