As the field of network science has dramatically expanded, researchers have discovered that many networked systems no longer operate in isolation. For example, communications systems often depend on the power-grid to be operational. As our world becomes more and more interconnected and interdependent due to modern technologies, the interactions between networks have taken on increasing importance. In this book, several of the founders and leading researchers in interactions between networks present the key findings in the field, including the possibility of cascading failures between systems, and provide a framework and methods for analyzing 'networks of networks. This book is an excellent introductory resource for scientists and graduate students in the field of complex networks in various disciplines.