As mentioned in the introduction of the previous chapter, the technological developments in space technology and observational techniques, during the last fifty years, have expanded the observer’s universe a hundredfold while the theorist has gone further beyond in trying to make sense of what one sees around her/him, by introducing a very high energy universe. The main question for which one is still groping for a definitive answer is ‘What is responsible for such high energy emissions’? Of course, there have been some answers most of which point towards one main agent – gravitation and accretion of matter by compact objects.