Magnetic fields are ubiquitous in the universe and their study forms an important area of research in modern day astrophysics. Starting from planetary magnetic fields like the one of the Earth, magnetic fields are found to exist in stars like the Sun, in galaxies to clusters of galaxies. The importance of magnetic fields for our understanding of the Universe has been appreciated since mid-1950's when the advent of radio astronomy and the studies of cosmic ray propagation and confinement revealed a Universe filled with thermal and relativistic plasma whose motions are controlled by magnetic fields. Indeed, cosmic magnetic fields play a crucial role in a number of physical processes in the universe from confining cosmic rays, controlling star formation, driving turbulence in accretion discs, to the production and collimation of jets in AGN's. An extensive review of astrophysical magnetic fields incorporating recent results are in Brandenburg and Subramanian(2005a). Let us first have a quick look into the estimates of the strength and coherence scales of magnetic fields in some astrophysical objects. The magnetic field of the Sun consists of large-scale and small-scale components. The magnetic field of the Sun consists of large-scale and small-scale components. The typical length scale associated with the large-scale field is the width of the toroidal flux belts with the same polarity around the 30? latitude corresponding to about 300 Mm (1Mm = 1000 km). The pressure scale height at the bottom of the convection zone is about 50 Mm and scales smaller than this is associated with the small-scale field.