A collection of sixteen coordinated reviews on the origins of large-scale magnetic fields in the Universe, this book discusses magnetic fields in all relevant astrophysical contexts, from the interstellar medium to the scales of galaxies and clusters of galaxies. Magnetic fields are described in their very diverse environments, from stellar winds to galactic haloes and astrophysical jets, together with the roles they play in forming the structures and shaping the dynamics of these objects. Both observational evidence and its theoretical interpretations are covered up to the largest scales in the Universe. The authors are all leading scientists in their fields, making this book an authoritative, up-to-date and enduring contribution to astrophysics. This volume is aimed at graduate students and researchers in astrophysics. Previously published in Space Science Reviews journal, Vol. 166/1-4 and Vol. 169/1-4, 2012.