The photonic lantern is an optoelectronic device emerging in recent years. It provides low-loss interfaces between single mode and multimode systems, with one side of the device being a multimode waveguide satisfying special designs, and the other side being a bunch of single mode waveguide. The theory and several typical structures are analyzed, and their characteristics and applications are summarized. The applications and research progress of the photonic lantern in astrophotonics, multiplexing, mode control, and high power laser are introduced, and its applications as a low-loss optical waveguide device in future optical systems is forecasted.