Dark energy' is the name given to the unknown cause of the Universe's accelerating expansion, which is one of the most significant and surprising discoveries in recent cosmology. Understanding this enigmatic ingredient of the Universe and its gravitational effects is a very active, and growing, field of research. In this volume, twelve world-leading authorities on the subject present the basic theoretical models that could explain dark energy, and the observational and experimental techniques employed to measure it. Covering the topic from its origin, through recent developments, to its future perspectives, this book provides a complete and comprehensive introduction to dark energy for a range of readers. It is ideal for physics graduate students who have just entered the field and researchers seeking an authoritative reference on the topic.