The nature and essence of Dark Matter and Dark Energy have become the central issue in modern cosmology over the past years as they make up most of the matter and energy content of the known universe. Accordingly, the subject matter is quite naturally tied to other fields such as astroparticle physics, quantum physics and general relativity. This extensive volume, an outgrowth of a topical and tutorial summer school, has been set up with the aim of constituting an advanced-level, multi-authored textbook which meets the needs of both postgraduate students and young researchers in the fields of modern cosmology and astrophysics.