One of the most common incipient losses of structural integrity in mechanical structures is the development and propagation of a crack. This happens also in rotating machinery and a very rich, but also in some way confusing, literature about cracked rotors has appeared in the last 30 years. In the paper, a general and wide overview about the behaviour of cracked rotors will be presented, covering several aspects of this topic. In particular modelling, dynamic behaviour and diagnostics of cracks will be analyzed in detail, introducing also practical examples and cases of industrial machinery, including some topics barely documented in literature such as helical and annular crack development