This book presents contributions on particle theory, quantum field theory, general relativity, quantum gravity, string theory and cosmology. These represent the outcome of numerous discussions, taking place among the authors over a period of several years, about the research landscape of present-day fundamental theoretical physics. Indeed, the resultant book documents their attempt, out of their respective and highly specialized scientific positions, to find a way of communicating about methods, achievements and promises of the different approaches which shape the development of this field. It is therefore also an attempt to elucidate the connections between these approaches, and present them not as disjoint ventures but rather as facets of a common quest for understanding. Readers interested in fundamental approaches of theoretical physics will find this book highly stimulating and enlightening.