An overview is given of observations of plasma waves in the heliosphere. To illustrate the range of phenomena involved, we discuss plasma waves in the solar wind and in the planetary magnetospheres as well as heliospheric radio emissions related to the boundary of the Solar System. Intensity of these radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere can be explained provided that the electron beams generating Langmuir waves exist in the post-shock plasma due to secondary shocks in the compressed solar wind beyond the termination shock. The field strengths of Langmuir waves required to generate the second harmonic emissions are 50–100 &mgr;V m−1. Alternatively, the emissions are generated in the vicinity of the heliopause. The Voyager 1 and 2 are proceeding toward a likely source region for Langmuir wave and these waves may be observed in situ in thenearfuture. ©1999 American Institute of Physics.