These lecture notes collect the material that I have been using over the years for various short courses on the physics of gravitational waves, first at the Institut d’Astrophysique de Paris (France), and then at SISSA (Italy) and various summer/winter schools. The level should be appropriate for PhD students in physics or for MSc students that have taken a first course in general relativity. I try as much as possible to derive results from first principles and focus on the physics, rather than on astrophysical applications. The reason is not only that the latter require a solid understanding of the physics, but it also lies in the trove of data that are being uncovered by the LIGO-Virgo-KAGRA collaboration after the first direct detection of gravitational waves. Any attempt to summarize such a rich and fast changing landscape and its evolving astrophysical interpretation is bound to become obsolete before the ink hits the page.