A conference on "Observational Evidence for Black Holes in the Universe" was held in Calcutta during January 10-17, 1998. This was the first time that experts had gathered to debate and discuss topics such as: Should black holes exist?; If so, how to detect them?; And Have we found them? This book is the essence of this gathering. Black holes are enigmatic objects since it is impossible to locate them through direct observations. State-of-the-art theoretical works and numerical simulations have given us enough clues of what to look for. Observations, from both ground and space-based missions, have been able to find these tell-tale signatures. This book is a compendium of our present knowledge about these theories and observations at the end of the 20th century. Combined, they give an idea of whether black holes, galactic as well as extragalactic, have been detected or not.