Written specifically for physicists and graduate students, this textbook focuses on fundamental and sometimes practical limitations on the ultimate performance that an astronomical system may reach, rather than presenting particular systems in detail. This second edition has been entirely restructured and almost doubled in size, in order to improve its clarity and to account for the great progress achieved in the last 15 years. It deals with ground-based and space-based astronomy and their respective fields. It presents the new generation of giant ground-based telescopes, with the new methods of optical interferometry and adaptive optics. But it also presents the ambitious concepts behind space missions aimed for the next decades. Avoiding particulars, it covers the whole of the electromagnetic spectrum, and touches upon the "new astronomies" becoming possible with gravitational waves and neutrinos.