This paper presents the state-of-the-art of a new class of microwave and mm-wave sources and intends to focus the attention of the microwave community on the new possibilities offered by them. Following a short introduction to the superconducting phenomena, the basic properties of Josephson tunnel junctions are presented. A description of the high frequency behavior of Josephson trasmission lines introduces the microwave and mm-wave emission mechanisms of Josephson oscillators. Performances, advantages and disadvantages of this new class of r.f. sources are discussed.